Isolating a bug?

FROM : Gregory Weston
DATE : Wed Apr 06 12:46:03 2005

From a program that's been shipping for about a year now without
stability complaints, I user sent a crash log that I find vexing. Why?
Because here's the call stack.

Exception:  EXC_BAD_ACCESS (0x0001)
Codes:      KERN_PROTECTION_FAILURE (0x0002) at 0x00000006

Thread 0 Crashed:
0  com.apple.CoreFoundation     0x901939cc CFHash + 0x18
1  com.apple.CoreFoundation     0x90192d28 __CFDictionaryFindBuckets1b +
0x70
2  com.apple.CoreFoundation     0x90192184 CFDictionaryGetValue + 0xec
3  com.apple.CoreFoundation     0x901b0fb0
_CFApplicationPreferencesCreateValueForKey + 0x6c
4  com.apple.Foundation         0x909fb38c -[NSUserDefaults
objectForKey:] + 0x98
5  my.program.signature         0x00008658 -[MyApplication
methodTakingAString:] + 0x5c

The routine identified in line 5 is called repeatedly (although not
rapidly) while the program is running - it had been invoked hundreds of
times without error during this session. The argument to objectForKey:
at line 4 is a freshly-created from NSString>>stringWithFormat:. I'm
not sure where to start looking for the cause of a problem that
manifests five invocations into external code. Advice?