diff --git a/src/app/core/persistence/operation-log/store/operation-log-compaction.service.spec.ts b/src/app/core/persistence/operation-log/store/operation-log-compaction.service.spec.ts
index e141fa7dc3..22d02df573 100644
--- a/src/app/core/persistence/operation-log/store/operation-log-compaction.service.spec.ts
+++ b/src/app/core/persistence/operation-log/store/operation-log-compaction.service.spec.ts
@@ -796,4 +796,206 @@ describe('OperationLogCompactionService', () => {
       expect(noteKeys.length).toBe(0);
     });
   });
+
+  // =========================================================================
+  // Race condition tests
+  // =========================================================================
+  // These tests verify behavior when concurrent operations occur during compaction.
+  // The compaction service uses locks to prevent data corruption, and these tests
+  // verify that the locking mechanism works correctly.
+
+  describe('race conditions', () => {
+    it('should request lock for every compact call', async () => {
+      // This test verifies that every compact() call requests the lock.
+      // The actual serialization happens in the LockService (tested separately in lock.service.spec.ts).
+      const lockRequests: string[] = [];
+
+      mockLockService.request.and.callFake(
+        async (_name: string, fn: () => Promise<void>) => {
+          lockRequests.push('lock-requested');
+          await fn();
+        },
+      );
+
+      // Call compact 3 times sequentially
+      await service.compact();
+      await service.compact();
+      await service.compact();
+
+      // Lock should have been requested 3 times
+      expect(lockRequests.length).toBe(3);
+    });
+
+    it('should always request lock with correct name', async () => {
+      await service.compact();
+
+      expect(mockLockService.request).toHaveBeenCalledWith(
+        'sp_op_log',
+        jasmine.any(Function),
+      );
+    });
+
+    it('should use lastSeq captured before deleteOpsWhere to protect new ops', async () => {
+      // Simulate a new operation being written after getLastSeq but before deleteOpsWhere
+      let capturedLastSeq: number | undefined;
+      let deleteFilterFn: ((entry: OperationLogEntry) => boolean) | undefined;
+
+      mockOpLogStore.getLastSeq.and.callFake(async () => {
+        capturedLastSeq = 100;
+        return 100;
+      });
+
+      mockOpLogStore.deleteOpsWhere.and.callFake(async (filterFn) => {
+        deleteFilterFn = filterFn;
+      });
+
+      await service.compact();
+
+      // Verify lastSeq was captured
+      expect(capturedLastSeq).toBe(100);
+
+      // Simulate a new operation written after getLastSeq (seq 101)
+      const newOpAfterSnapshot: OperationLogEntry = {
+        seq: 101, // Written after getLastSeq returned 100
+        op: {} as any,
+        appliedAt: Date.now() - COMPACTION_RETENTION_MS - 1000, // Old enough to delete
+        source: 'local',
+        syncedAt: Date.now() - COMPACTION_RETENTION_MS - 500, // Synced
+      };
+
+      // The filter should NOT delete this op because seq > lastSeq
+      expect(deleteFilterFn!(newOpAfterSnapshot)).toBeFalse();
+
+      // But an op with seq <= lastSeq and old enough should be deleted
+      const oldOpBeforeSnapshot: OperationLogEntry = {
+        seq: 99,
+        op: {} as any,
+        appliedAt: Date.now() - COMPACTION_RETENTION_MS - 1000,
+        source: 'remote',
+        syncedAt: Date.now() - COMPACTION_RETENTION_MS - 500,
+      };
+      expect(deleteFilterFn!(oldOpBeforeSnapshot)).toBeTrue();
+    });
+
+    it('should handle concurrent compact and emergencyCompact calls', async () => {
+      const callOrder: string[] = [];
+
+      mockLockService.request.and.callFake(
+        async (_name: string, fn: () => Promise<void>) => {
+          callOrder.push('lock-start');
+          await fn();
+          callOrder.push('lock-end');
+        },
+      );
+
+      // Launch both regular and emergency compaction concurrently
+      const [regularResult, emergencyResult] = await Promise.all([
+        service.compact().then(() => 'regular-done'),
+        service
+          .emergencyCompact()
+          .then((success) => (success ? 'emergency-done' : 'emergency-failed')),
+      ]);
+
+      // Both should complete
+      expect(regularResult).toBe('regular-done');
+      expect(emergencyResult).toBe('emergency-done');
+
+      // Lock should have been acquired twice (serialized)
+      expect(callOrder.filter((c) => c === 'lock-start').length).toBe(2);
+    });
+
+    it('should not delete operations that arrive during compaction', async () => {
+      // This test verifies the TOCTOU protection:
+      // 1. getLastSeq() returns 100
+      // 2. New op with seq 101 arrives (simulated)
+      // 3. deleteOpsWhere should NOT delete seq 101
+
+      const operationsInStore: OperationLogEntry[] = [
+        {
+          seq: 99,
+          op: { id: 'op-99' } as any,
+          appliedAt: Date.now() - COMPACTION_RETENTION_MS - 2000,
+          source: 'remote',
+          syncedAt: Date.now() - COMPACTION_RETENTION_MS - 1000,
+        },
+        {
+          seq: 100,
+          op: { id: 'op-100' } as any,
+          appliedAt: Date.now() - COMPACTION_RETENTION_MS - 1000,
+          source: 'remote',
+          syncedAt: Date.now() - COMPACTION_RETENTION_MS - 500,
+        },
+        // This op "arrives" during compaction (seq > lastSeq when getLastSeq was called)
+        {
+          seq: 101,
+          op: { id: 'op-101' } as any,
+          appliedAt: Date.now() - COMPACTION_RETENTION_MS - 500,
+          source: 'local',
+          syncedAt: Date.now() - COMPACTION_RETENTION_MS - 100,
+        },
+      ];
+
+      mockOpLogStore.getLastSeq.and.returnValue(Promise.resolve(100));
+
+      const deletedSeqs: number[] = [];
+      mockOpLogStore.deleteOpsWhere.and.callFake(async (filterFn) => {
+        for (const entry of operationsInStore) {
+          if (filterFn(entry)) {
+            deletedSeqs.push(entry.seq);
+          }
+        }
+      });
+
+      await service.compact();
+
+      // seq 99 and 100 should be eligible for deletion (old, synced, seq <= lastSeq)
+      expect(deletedSeqs).toContain(99);
+      expect(deletedSeqs).toContain(100);
+
+      // seq 101 should NOT be deleted (seq > lastSeq at time of check)
+      expect(deletedSeqs).not.toContain(101);
+    });
+
+    it('should complete all steps atomically within the lock', async () => {
+      // Verify that if saveStateCache fails, subsequent steps don't run
+      const callOrder: string[] = [];
+
+      mockLockService.request.and.callFake(
+        async (_name: string, fn: () => Promise<void>) => {
+          callOrder.push('lock-acquired');
+          try {
+            await fn();
+          } finally {
+            callOrder.push('lock-released');
+          }
+        },
+      );
+
+      mockOpLogStore.saveStateCache.and.callFake(async () => {
+        callOrder.push('saveStateCache');
+        throw new Error('Simulated failure');
+      });
+
+      mockOpLogStore.resetCompactionCounter.and.callFake(async () => {
+        callOrder.push('resetCompactionCounter');
+      });
+
+      mockOpLogStore.deleteOpsWhere.and.callFake(async () => {
+        callOrder.push('deleteOpsWhere');
+      });
+
+      await expectAsync(service.compact()).toBeRejectedWithError('Simulated failure');
+
+      // Lock should have been acquired and released
+      expect(callOrder).toContain('lock-acquired');
+      expect(callOrder).toContain('lock-released');
+
+      // saveStateCache was called
+      expect(callOrder).toContain('saveStateCache');
+
+      // But subsequent steps should NOT have been called
+      expect(callOrder).not.toContain('resetCompactionCounter');
+      expect(callOrder).not.toContain('deleteOpsWhere');
+    });
+  });
 });
diff --git a/src/app/core/persistence/operation-log/store/operation-log-hydrator.service.spec.ts b/src/app/core/persistence/operation-log/store/operation-log-hydrator.service.spec.ts
index 787737ecfd..88d39e5d93 100644
--- a/src/app/core/persistence/operation-log/store/operation-log-hydrator.service.spec.ts
+++ b/src/app/core/persistence/operation-log/store/operation-log-hydrator.service.spec.ts
@@ -503,6 +503,151 @@ describe('OperationLogHydratorService', () => {
 
         expect(mockSchemaMigrationService.migrateOperations).toHaveBeenCalled();
       });
+
+      // Additional version mismatch tests
+
+      it('should handle snapshot with version newer than current (future version)', async () => {
+        // This scenario can happen if a user downgrades the app
+        const futureSnapshot = createMockSnapshot({
+          schemaVersion: CURRENT_SCHEMA_VERSION + 5, // Future version
+        });
+        mockOpLogStore.loadStateCache.and.returnValue(Promise.resolve(futureSnapshot));
+        // Future version doesn't need migration (migration is only for older versions)
+        mockSchemaMigrationService.needsMigration.and.returnValue(false);
+
+        // Should not throw, just load the data
+        await service.hydrateStore();
+
+        expect(mockStore.dispatch).toHaveBeenCalledWith(
+          loadAllData({ appDataComplete: mockState }),
+        );
+      });
+
+      it('should migrate snapshot and operations together when both need migration', async () => {
+        // Both snapshot and tail operations have old schema version
+        const oldSnapshot = createMockSnapshot({
+          schemaVersion: 0,
+          lastAppliedOpSeq: 5,
+        });
+        const migratedSnapshot = createMockSnapshot({
+          schemaVersion: CURRENT_SCHEMA_VERSION,
+          lastAppliedOpSeq: 5,
+        });
+
+        const tailOps = [
+          createMockEntry(
+            6,
+            createMockOperation('op-6', OpType.Update, { schemaVersion: 0 }),
+          ),
+          createMockEntry(
+            7,
+            createMockOperation('op-7', OpType.Update, { schemaVersion: 0 }),
+          ),
+        ];
+
+        mockOpLogStore.loadStateCache.and.returnValue(Promise.resolve(oldSnapshot));
+        mockOpLogStore.getOpsAfterSeq.and.returnValue(Promise.resolve(tailOps));
+        mockSchemaMigrationService.needsMigration.and.returnValue(true);
+        mockSchemaMigrationService.migrateStateIfNeeded.and.returnValue(migratedSnapshot);
+        mockSchemaMigrationService.operationNeedsMigration.and.returnValue(true);
+
+        const migratedOps = tailOps.map((e) => ({
+          ...e.op,
+          schemaVersion: CURRENT_SCHEMA_VERSION,
+        }));
+        mockSchemaMigrationService.migrateOperations.and.returnValue(migratedOps);
+
+        await service.hydrateStore();
+
+        // Both snapshot and operations should be migrated
+        expect(mockSchemaMigrationService.migrateStateIfNeeded).toHaveBeenCalled();
+        expect(mockSchemaMigrationService.migrateOperations).toHaveBeenCalled();
+        // Operations should be applied via applier service
+        expect(mockOperationApplierService.applyOperations).toHaveBeenCalled();
+      });
+
+      it('should handle mixed schema versions in tail operations', async () => {
+        // Some tail ops are old version, some are current
+        const snapshot = createMockSnapshot({
+          schemaVersion: CURRENT_SCHEMA_VERSION,
+          lastAppliedOpSeq: 5,
+        });
+
+        const tailOps = [
+          createMockEntry(
+            6,
+            createMockOperation('op-6', OpType.Update, { schemaVersion: 0 }), // Old
+          ),
+          createMockEntry(
+            7,
+            createMockOperation('op-7', OpType.Update, {
+              schemaVersion: CURRENT_SCHEMA_VERSION,
+            }), // Current
+          ),
+          createMockEntry(
+            8,
+            createMockOperation('op-8', OpType.Update, { schemaVersion: 0 }), // Old
+          ),
+        ];
+
+        mockOpLogStore.loadStateCache.and.returnValue(Promise.resolve(snapshot));
+        mockOpLogStore.getOpsAfterSeq.and.returnValue(Promise.resolve(tailOps));
+        mockSchemaMigrationService.needsMigration.and.returnValue(false);
+        // At least one operation needs migration
+        mockSchemaMigrationService.operationNeedsMigration.and.callFake(
+          (op: Operation) => op.schemaVersion === 0,
+        );
+
+        await service.hydrateStore();
+
+        // migrateOperations should be called since some ops need migration
+        expect(mockSchemaMigrationService.migrateOperations).toHaveBeenCalled();
+      });
+
+      it('should not migrate operations if none need migration', async () => {
+        const snapshot = createMockSnapshot({
+          schemaVersion: CURRENT_SCHEMA_VERSION,
+          lastAppliedOpSeq: 5,
+        });
+
+        const tailOps = [
+          createMockEntry(
+            6,
+            createMockOperation('op-6', OpType.Update, {
+              schemaVersion: CURRENT_SCHEMA_VERSION,
+            }),
+          ),
+        ];
+
+        mockOpLogStore.loadStateCache.and.returnValue(Promise.resolve(snapshot));
+        mockOpLogStore.getOpsAfterSeq.and.returnValue(Promise.resolve(tailOps));
+        mockSchemaMigrationService.needsMigration.and.returnValue(false);
+        mockSchemaMigrationService.operationNeedsMigration.and.returnValue(false);
+
+        await service.hydrateStore();
+
+        // migrateOperations should NOT be called
+        expect(mockSchemaMigrationService.migrateOperations).not.toHaveBeenCalled();
+      });
+
+      it('should handle undefined schemaVersion in snapshot (legacy data)', async () => {
+        // Legacy data from before schema versioning was introduced
+        const legacySnapshot = createMockSnapshot({ schemaVersion: undefined });
+        const migratedSnapshot = createMockSnapshot({
+          schemaVersion: CURRENT_SCHEMA_VERSION,
+        });
+
+        mockOpLogStore.loadStateCache.and.returnValue(Promise.resolve(legacySnapshot));
+        mockSchemaMigrationService.needsMigration.and.returnValue(true);
+        mockSchemaMigrationService.migrateStateIfNeeded.and.returnValue(migratedSnapshot);
+
+        await service.hydrateStore();
+
+        // Should call migration for legacy (undefined version) snapshot
+        expect(mockSchemaMigrationService.migrateStateIfNeeded).toHaveBeenCalledWith(
+          legacySnapshot,
+        );
+      });
     });
 
     describe('backup recovery', () => {
diff --git a/src/app/core/persistence/operation-log/sync/operation-log-download.service.spec.ts b/src/app/core/persistence/operation-log/sync/operation-log-download.service.spec.ts
index 0d49e2b886..0b71949ec5 100644
--- a/src/app/core/persistence/operation-log/sync/operation-log-download.service.spec.ts
+++ b/src/app/core/persistence/operation-log/sync/operation-log-download.service.spec.ts
@@ -636,22 +636,134 @@ describe('OperationLogDownloadService', () => {
         expect(result.newOps[0].id).toBe('op-2');
       });
 
-      // NOTE: These tests are skipped because the service uses real setTimeout
-      // with exponential backoff delays (1s + 2s + 4s = 7s) that exceed test timeouts.
-      // jasmine.clock() doesn't work with Promise-based async code.
-      // The functionality is still tested manually - these tests verify retry behavior.
+      // Download failure handling tests
+      // These tests verify retry behavior by counting download attempts and verifying
+      // the final result, without waiting for real retry delays.
       describe('download failure handling', () => {
-        it('should handle file download failures gracefully', () => {
-          // Test is skipped - retry delays cause timeout
-          // The service correctly handles failures and returns { success: false, failedFileCount: N }
-          pending('Skipped due to retry delays exceeding test timeout');
-        });
+        it('should return success:false when file download fails after all retries', async () => {
+          // Setup: provider that always fails to download
+          let downloadAttempts = 0;
+          mockManifestService.loadRemoteManifest.and.returnValue(
+            Promise.resolve({
+              operationFiles: ['ops/ops_client_fail.json'],
+              version: 1,
+            }),
+          );
 
-        it('should notify user about failed downloads', () => {
-          // Test is skipped - retry delays cause timeout
-          // The service correctly shows snackbar notification for failed downloads
-          pending('Skipped due to retry delays exceeding test timeout');
-        });
+          // Mock downloadFile to track attempts and always reject
+          // The service will retry MAX_DOWNLOAD_RETRIES (3) times
+          mockFileProvider.downloadFile.and.callFake(async () => {
+            downloadAttempts++;
+            throw new Error('Network error');
+          });
+
+          // Act: Download with retry (this will take ~7 seconds with real delays)
+          // We're testing the behavior, not the timing
+          const result = await service.downloadRemoteOps(mockFileProvider);
+
+          // Assert: Should have attempted download multiple times
+          // Note: 1 initial attempt + MAX_DOWNLOAD_RETRIES (3) = 4 total attempts
+          expect(downloadAttempts).toBe(4);
+
+          // Assert: Result should indicate failure
+          expect(result.success).toBeFalse();
+          expect(result.failedFileCount).toBe(1);
+          expect(result.newOps).toEqual([]);
+        }, 30000); // Extended timeout for retry delays
+
+        it('should notify user about failed downloads via snackbar', async () => {
+          mockManifestService.loadRemoteManifest.and.returnValue(
+            Promise.resolve({
+              operationFiles: ['ops/ops_client_fail.json'],
+              version: 1,
+            }),
+          );
+
+          // Mock downloadFile to always fail
+          mockFileProvider.downloadFile.and.rejectWith(new Error('Network error'));
+
+          // Act
+          await service.downloadRemoteOps(mockFileProvider);
+
+          // Assert: Should have shown error notification
+          expect(mockSnackService.open).toHaveBeenCalledWith(
+            jasmine.objectContaining({
+              type: 'ERROR',
+            }),
+          );
+        }, 30000); // Extended timeout for retry delays
+
+        it('should succeed if download succeeds after retry', async () => {
+          let downloadAttempts = 0;
+          const mockOps = [
+            {
+              seq: 1,
+              op: {
+                id: 'op-retry-success',
+                actionType: '[Test] Action',
+                opType: OpType.Create,
+                entityType: 'TASK',
+                entityId: 'task-1',
+                payload: {},
+                clientId: 'remoteClient',
+                vectorClock: { remoteClient: 1 },
+                timestamp: Date.now(),
+                schemaVersion: 1,
+              },
+              appliedAt: Date.now(),
+              source: 'remote' as const,
+            },
+          ];
+
+          mockManifestService.loadRemoteManifest.and.returnValue(
+            Promise.resolve({
+              operationFiles: ['ops/ops_client_retry.json'],
+              version: 1,
+            }),
+          );
+          mockOpLogStore.getAppliedOpIds.and.returnValue(Promise.resolve(new Set()));
+
+          // Fail first 2 attempts, succeed on 3rd
+          mockFileProvider.downloadFile.and.callFake(async () => {
+            downloadAttempts++;
+            if (downloadAttempts < 3) {
+              throw new Error('Temporary network error');
+            }
+            return { dataStr: JSON.stringify(mockOps), rev: 'test-rev' };
+          });
+
+          // Act
+          const result = await service.downloadRemoteOps(mockFileProvider);
+
+          // Assert: Should have retried and eventually succeeded
+          expect(downloadAttempts).toBe(3);
+          expect(result.success).toBeTrue();
+          expect(result.newOps.length).toBe(1);
+          expect(result.newOps[0].id).toBe('op-retry-success');
+        }, 30000); // Extended timeout for retry delays
+
+        it('should log warnings for each retry attempt', async () => {
+          mockManifestService.loadRemoteManifest.and.returnValue(
+            Promise.resolve({
+              operationFiles: ['ops/ops_client_warn.json'],
+              version: 1,
+            }),
+          );
+
+          // Fail always to trigger all retries
+          mockFileProvider.downloadFile.and.rejectWith(new Error('Network error'));
+
+          // Act
+          await service.downloadRemoteOps(mockFileProvider);
+
+          // Assert: Should have logged warnings for each failed attempt
+          // Total warnings = one warning per failed attempt that will be retried (3)
+          // plus one final warning when all retries exhausted (1) = 4 total
+          // Actually looking at the code: warnings are logged before each retry delay,
+          // so it's 3 warnings (one before each of the 3 retries)
+          // The 4th call might be from the final failure log
+          expect((OpLog.warn as jasmine.Spy).calls.count()).toBeGreaterThanOrEqual(3);
+        }, 30000); // Extended timeout for retry delays
       });
 
       it('should return success true when all files download successfully', async () => {