Sometimes you are running, and you get an error like this:
a.out(11233) malloc: *** error for object 0x100120: double free *** set a breakpoint in malloc_error_break to debug
Usually by the time you can react, your program has moved far past that. As suggested, you can set a breakpoint on malloc_error_break to catch the offending free().
Another even more common occurrence in Cocoa programming is this error:
2010-01-20 20:03:38.080 badmessage[11303:10b] *** -[NSCFArray frobulate:]: unrecognized selector sent to ...