I just realized for my cocoa application I can set the garbage collector setting for the target but also for the project.
My app did crash since I activated GB only in the project properties not in the target properties like I used to do. Changes in the project settings did not apply to the target settings. What use is the Garbage Collector in the project settings?
You need to understand a bit more about how Xcode settings work.
They're basically a hierarchy and at each level, you can either have a value defined, in which case it overrides the parent setting, or not, in which case the parent's setting is used.
What's probably happened in your case, is that even though you've defined it at the project level, there's a target level setting that says not to use garbage collection.
If a setting is defined at a particular level, it will be displayed in bold. To undefine a setting at a particular level, there's a button at the bottom left of the Info window that you can use.