Once you have cleaned up enough leaks (or possibly even before), you will likely run into cases in which objects are deallocated prematurely rather than begin leaked. This can result in errors from the ObjC runtime, application crashes or very bizarre application behaviour. Fortunately, OmniObjectAlloc can use some of the builtin functionality of Foundation to locate prematurely released objects.
On the main document window, click on the 'Settings...' button.
The 'Enable Zombies' checkbox can be checked before you start you application to automatically set the environement variables NSZombieEnabled=YES and NSDeallocateZombies=NO (these environment variables can be used in the debugger routinely to detect 'zombies'). This option obviously cause your application to run more slowly and consume more memory, so you may not want to use it all the time. Curently this option is only checked at startup time (since it is transmitted via environment variables). At some point in the future, this restriction may be removed.
Once you get your application close to the error, you should deselect the 'Forget Deallocated Objects' checkbox in the document window. You must do this before the object is deallocated, not just before you hit the zombie.
When you send a message to a deallocated object that has been turned into a zombie, the ObjC runtime will point a message stating the address and class of the object. You can then select the class and object in the document. The process from this point is very similar to determining the cause of a leak. The problem in this case is that the object was release too soon. There will be a matching number of retain/release events (since any extra release events that would have been sent will fail due to the object having been turned into a zombie). The list of events or the EventMatching panel may be used to group the matching events. Eventually, you will find a release event that does not have a matching retain. Depending upon the circumstances, you will either need to add a retain or remove a release to correct the problem.