NSZombieEnabled is an environment variable which controls whether the Foundation runtime will use zombies. When zombies are enabled, a deallocated object’s class is dynamically changed to be _NSZombie, and by default, the memory region is never marked as free, although this can be controlled separately. The end result is that, with zombies enabled, messages to deallocated objects will no longer behave strangely or crash in difficult-to-understand ways, but will instead log a message and die in a predictable and debugger-breakpointable way. This is the tool to use when trying to track down over-releases and premature releases.