In Xcode 4.x press

⌥⌘R

(or click Menubar > Product > Scheme > Edit Scheme)

select the "Diagnostics" tab and click "Enable Zombie Objects":

This turns released objects into NSZombie instances that print console warnings when used again. This is a debugging aid that increases memory use (no object is really released) but improves error reporting.

A typical case is when you over-release an object and you don't know which one:

• With zombies: -[UITableView release]: message sent to deallocated instance
• Without zombies:

This Xcode setting is ignored when you archive the application for App Store submission. You don't need to touch anything before releasing your application.

Pressing ⌥⌘R is the same as selecting Product > Run while keeping the Alt key pressed.
Clicking the "Enable Zombie Objects" checkbox is the same as manually adding "NSZombieEnabled = YES" in the section "Environment Variables" of the tab Arguments.

Jano's answer is the easiest way to find it.. another way would be if you click on the scheme drop down bar -> edit scheme -> arguments tab and then add NSZombieEnabled in the Environment Variables column and YES in the value column...

I find this alternative more convenient:

1. Click the "Run Button Dropdown"
2. From the list choose Profile
3. The program "Instruments" should open where you can also choose Zombies
4. Now you can interact with your app and try to cause the error
5. As soon as the error happens you should get a hint on when your object was released and therefore deallocated.

As soon as a zombie is detected you then get a neat "Zombie Stack" that shows you when the object in question was allocated and where it was retained or released:

Event Type    RefCt     Responsible CallerMalloc            1     -[MyViewController loadData:]Retain            2     -[MyDataManager initWithBaseURL:]Release           1     -[MyDataManager initWithBaseURL:]Release           0     -[MyViewController loadData:]Zombie           -1     -[MyService prepareURLReuqest]

Advantages compared to using the diagnostic tab of the Xcode Schemes:

1. If you forget to uncheck the option in the diagnostic tab there no objects will be released from memory.

2. You get a more detailed stack that shows you in what methods your corrupt object was allocated / released or retained.