At the risk of sounding obvious, always check the spelling of your forward class files. Sometimes XCode (at least XCode 4.3.2) will turn a declaration green that's actually camel cased incorrectly. Like in this example:
"_OBJC_CLASS_$_RadioKit", referenced from:
objc-class-ref in RadioPlayerViewController.o
If RadioKit was a class file and you make it a property of another file, in the interface declaration, you might see that
Radiokit *rk;
has "Radiokit" in green when the actual decalaration should be:
RadioKit *rk;
This error will also throw this type of error. Another example (in my case), is when you have _iPhone and _iphone extensions on your class names for universal apps. Once I changed the appropriate file from _iphone to the correct _iPhone, the errors went away.
