What makes this an insidious problem is that sometimes those crashes happen at random, are not always reproducible, and the stack trace you get doesn't offer clear clues of what portion of the code is at fault. As you can see from the picture below, all code in the stack trace is system code, none in your control:
Sometimes the stack trace is slightly different, here's another one:
The presence of _CFAutoreleasePoolPop in the trace is a strong indication of the fact that an object that's already been deallocated, as a result of over-release, is now attempted to be released again, during the draining of an autorelease pool.
In such cases you should be looking for 2 pieces of information. What's the address of the object being released, its invalid pointer, and where did that pointer come from in the first place, that is, where in your code it was first allocated. Getting to know the variable name in your code that holds that offending pointer, as it appears at the point of allocation, helps tremendously in solving the issue. All you need to do is search in your code where that variable is being used, where it gets released, and so on. Often times you'll find that you're releasing an object that's already autoreleased (it hasn't been created with a call to a method that starts with "alloc" or "new").
You should be able to somehow reproduce the problem, if you can do that, you're good to go. If reproducing the issue is hard, you might want to try placing breakpoints that stop the execution for a while. In some instances that alters the timing of execution in the system and helps reproducing the issue. Sometimes your only recourse is to run the same code repeatedly until it crashes.
Once you can reproduce the crash, the first thing you need to do is set some environment variables to aid in debugging. In the left pane of Xcode, locate the executable group, and double-click the name of the executable for your project:
Choose the "Arguments" tab in the window that opens and set the NSZombieEnabled, NSDebugEnabled, NSAutoreleaseFreedObjectCheckEnabled and MallocStackLoggingNoCompact like so:
Then run your code, if it crashes, look in the debugger console and you should see something like this:
[Session started at 2011-05-20 11:14:57 +0300.]
GNU gdb 6.3.50-20050815 (Apple version gdb-1510) (Wed Sep 22 02:45:02 UTC 2010)
Copyright 2004 Free Software Foundation, Inc.
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This GDB was configured as "x86_64-apple-darwin".sharedlibrary apply-load-rules all
Attaching to process 40114.
PubPit(40114) malloc: recording malloc stacks to disk using standard recorder
PubPit(40114) malloc: stack logging compaction turned off; size of log files on disk can increase rapidly
PubPit(40114) malloc: process 40076 no longer exists, stack logs deleted from /tmp/stack-logs.40076.PubPit.p5tTh9.index
PubPit(40114) malloc: stack logs being written into /tmp/stack-logs.40114.PubPit.Vpmlum.index
2011-05-20 11:14:59.890 PubPit[40114:207] *** -[CFDictionary release]: message sent to deallocated instance 0x652e280
Data Formatters temporarily unavailable, will re-try after a 'continue'. (Not safe to call dlopen at this time.)
The key info here is:
*** -[CFDictionary release]: message sent to deallocated instance 0x652e280
In my case I have an instance of CFDictionary that was initially allocated at address 0x652e280, which now is deallocated once too many times. The question is where did I ever allocate that object. To find that I do:
(gdb) info malloc-history 0x652e280
Alloc: Block address: 0x0652e280 length: 48
Stack - pthread: 0xa057c540 number of frames: 42
0: 0x949d617c in malloc_zone_malloc
1: 0x132080d in _CFRuntimeCreateInstance
2: 0x132040a in CFBasicHashCreate
3: 0x1321ebb in __CFDictionaryCreateGeneric
4: 0x1321bd1 in CFDictionaryCreateMutable
5: 0x13462ac in parseXMLElement
6: 0x13469fe in parsePListTag
7: 0x1345891 in parseXMLElement
8: 0x1344a94 in _CFPropertyListCreateFromXMLStringError
9: 0x13435a8 in _CFPropertyListCreateWithData
10: 0x1399957 in _CFPropertyListCreateFromXMLData
11: 0xf92ef in +[NSPropertyListSerialization propertyListFromData:mutabilityOption:format:errorDescription:]
12: 0xfa9c in -[Model loadData] at /Users/bhapca/WORK/Cocoa/Pubpit/Classes/Model.m:171
13: 0x102b8 in -[Model init] at /Users/bhapca/WORK/Cocoa/Pubpit/Classes/Model.m:251
14: 0x103fe in +[Model sharedModel] at /Users/bhapca/WORK/Cocoa/Pubpit/Classes/Model.m:268
15: 0x6c54 in -[PagingViewController loadView] at /Users/bhapca/WORK/Cocoa/Pubpit/Classes/PagingViewController.m:716
16: 0x4275e3 in -[UIViewController view]
17: 0x425a57 in -[UIViewController contentScrollView]
18: 0x436201 in -[UINavigationController _computeAndApplyScrollContentInsetDeltaForViewController:]
19: 0x434831 in -[UINavigationController _layoutViewController:]
20: 0x435b4c in -[UINavigationController _startTransition:fromViewController:toViewController:]
21: 0x430606 in -[UINavigationController _startDeferredTransitionIfNeeded]
22: 0x548e01 in -[UILayoutContainerView layoutSubviews]
23: 0xd6c451 in -[CALayer layoutSublayers]
24: 0xd6c17c in CALayerLayoutIfNeeded
25: 0xd6537c in _ZN2CA7Context18commit_transactionEPNS_11TransactionE
26: 0xd650d0 in _ZN2CA11Transaction6commitEv
27: 0x37c19f in -[UIApplication _reportAppLaunchFinished]
28: 0x37c659 in -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:]
29: 0x386db2 in -[UIApplication handleEvent:withNewEvent:]
30: 0x37f202 in -[UIApplication sendEvent:]
31: 0x384732 in _UIApplicationHandleEvent
32: 0x1cdda36 in PurpleEventCallback
33: 0x13f2064 in __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__
34: 0x13526f7 in __CFRunLoopDoSource1
35: 0x134f983 in __CFRunLoopRun
36: 0x134f240 in CFRunLoopRunSpecific
37: 0x134f161 in CFRunLoopRunInMode
38: 0x37bfa8 in -[UIApplication _run]
39: 0x38842e in UIApplicationMain
40: 0x1e74 in main at /Users/bhapca/WORK/Cocoa/Pubpit/main.m:14
41: 0x1e05 in start
BINGO ! My code that allocates that object and the variable used to point to that object is in: -[Model loadData] at /Users/bhapca/WORK/Cocoa/Pubpit/Classes/Model.m:171
Now all I need is too look at that code and figure out where that object gets released more than needed.
It's important to remove those NSZombieEnabled, NSDebugEnabled, NSAutoreleaseFreedObjectCheckEnabled and MallocStackLoggingNoCompact environment variables, once you're done with debugging, to prevent them to affect the released version of your app.
Credit goes to this site, where I first learned about MallocStackLoggingNoCompact.
Update: Apparently the same investigation can be performed using the Instruments tool that comes with Xcode 4, by choosing the Zombies trace template. I believe that is the method now recommended by Apple for dealing with this type of issues.