Sometimes Xcode would not compile your project in its latest state. This is most obvious when you move files around: rename images or remove .xib files. Your app would crash randomly for some weird reason, because Xcode is still using that .xib file you just deleted and tries to initialize it.

In order to fix these caching kind of bugs in Xcode, you have to go to the Derived Data folder and delete everything that’s inside (but not the folder itself). In Finder, just use Go ? Go to Folder... and enter:

~/Library/Developer/Xcode/DerivedData

Close Xcode and clear this folder. That’s it, after reindexing your project will be good to go.

P.S. This happened most often with Xcode 5, now that 6 is out, maybe that’s fixed, but it’s still useful to remember that just in case.