I saw other posts about the error:
Unknown class MyOldClass in Interface Builder file.
It happened to me when i deleted "MyOldClass" and added a new class for the nib's file owner. I also made sure 'MyOldClass' was not in the targets and my new class was. I also tried going to Build->Clean All Targets. I still get that error. How does xcode still have reference to MyOldClass?