I have a garbage collected Cocoa application built on 10.5 frameworks. In an NSOperation In a loop I am quickly creating hundreds of NSManagedObjects. Frequently the creation of those NSManagedObejcts will crash with a exc_bad_access error. for (offsetCount; offsetCount < [parsedData count]; offsetCount++) { NSManagedObject *child = [NSEntityDescription insertNewObjectForEntityForName:@"Thread" inManagedObjectContext:[self moc]]; Thumbnail *thumb = [Thumbnail insertInManagedObjectContext:[self moc]]; Image *image = [Image insertInManagedObjectContext:[self moc]]; ... } Thumbnail and Image are both subclasses of NSManagedObject generated with mogenerator. insertInManagedObjectContext: looks like NSParameterAssert(moc_); return [NSEntityDescription insertNewObjectForEntityForName:@"Thumbnail" inManagedObjectContext:moc_]; NSParameterAssert(moc_); return [NSEntityDescription insertNewObjectForEntityForName:@"Image" inManagedObjectContext:moc_]; The NSManagedObjectContext returned by [self moc] is created for the NSOperation with NSPersistentStoreCoordinator *coord = [(MyApp_AppDelegate *)[[NSApplication sharedApplication] delegate] persistentStoreCoordinator]; self.moc = [[NSManagedObjectContext alloc] init]; [self.moc setPersistentStoreCoordinator:coord]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(contextDidSave:) name:NSManagedObjectContextDidSaveNotification object:self.moc]; [self.moc setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy]; [self.moc setUndoManager:nil]; [self.moc setRetainsRegisteredObjects:YES]; moc is defined as (nonatomic, retain) and synthesized. As far as I can tell it, the persistent store and my appDelegate have no reason to be and are not being garbage collected. The stack trace looks like Thread 2 Crashed: Dispatch queue: com.apple.root.default-priority 0 libauto.dylib 0x00007fff82d63600 auto_zone_root_write_barrier + 688 1 libobjc.A.dylib 0x00007fff826f963b objc_assign_strongCast_gc + 59 2 com.apple.CoreFoundation 0x00007fff88677068 __CFBasicHashAddValue + 504 3 com.apple.CoreFoundation 0x00007fff88676d2f CFBasicHashAddValue + 191 4 com.apple.CoreData 0x00007fff82bdee5e -[NSManagedObjectContext(_NSInternalAdditions) _insertObjectWithGlobalID:globalID:] + 190 5 com.apple.CoreData 0x00007fff82bded24 -[NSManagedObjectContext insertObject:] + 148 6 com.apple.CoreData 0x00007fff82bbd75c -[NSManagedObject initWithEntity:insertIntoManagedObjectContext:] + 716 7 com.apple.CoreData 0x00007fff82bdf075 +[NSEntityDescription insertNewObjectForEntityForName:inManagedObjectContext:] + 101 8 com.yourcompany.MyApp 0x000000010002c7a7 +[_Thumbnail insertInManagedObjectContext:] + 256 (_Thumbnail.m:14) 9 com.yourcompany.MyApp 0x000000010002672d -[ThreadParse main] + 10345 (B4ChanThreadParse.m:174) 10 com.apple.Foundation 0x00007fff85ee807e -[__NSOperationInternal start] + 698 11 com.apple.Foundation 0x00007fff85ee7d23 ____startOperations_block_invoke_2 + 99 12 libSystem.B.dylib 0x00007fff812bece8 _dispatch_call_block_and_release + 15 13 libSystem.B.dylib 0x00007fff8129d279 _dispatch_worker_thread2 + 231 14 libSystem.B.dylib 0x00007fff8129cbb8 _pthread_wqthread + 353 15 libSystem.B.dylib 0x00007fff8129ca55 start_wqthread + 13 My app is crashing in other places with exc_bad_access but this is code that it happens most with. All of the stack traces look similar and have something to do with CFHash. Any help would be appreciated.