If you're doing iPhone development, setting NSZombieEnabled to YES is critical to debugging. It allows you to see what deallocated instances received a message. If this is not enabled, you won't have any idea why your app crashed. With it enabled, at least you know you sent a message to a deallocated instance, say, of an NSArray. But you have to be careful with this, because it prevents all objects from ever being deallocated! Make sure to turn it off before releasing your app.
Further reading here.
