If you tried to debug your app with zombies enabled under ARC, you may have noticed that you don’t get zombies anymore. That’s most probably because of a bug in the Foundation framework that affects iOS 5 and OS X 10.7. This bug prevents ARC from "cleaning up" instance variables at deallocation-time according to Apple Technical Q&A QA1758. Apple strongly encourages you to run your app on an iOS 6+, or OS X 10.8+ system when debugging with Zombies.
In the meantime, you can workaround this bug if you are using ARC and trying to debug zombies on older systems. Just compile this NSObject+ARCZombie.m file in your project and ivars will be automatically deallocated under ARC when zombies are enabled, as it should be.
Now you might think Hey, you are swizzling dealloc, isn't that dangerous? Well, yes it is, but note that the dealloc method is only swizzled when the NSZombieEnabled environment variable is enabled, so even if this code slips in your release build, il will cause no harm at all.
UPDATE: After I posted this, Greg Parker warned me on twitter:
@0xced That'll cause over-release of associated objects on some systems.
So use this workaround with care and remember that the best solution is what Apple recommends: run your app on an iOS 6+ or OS X 10.8+ system when debugging with Zombies.