I have an iPhone app that uses the json-framework. I moved some of the code, including the json-framework source, from the main project to a static library. When I did this, the json-framework stopped getting compiled into the binary (double checked with class dump). This causes a nasty error:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSCFString JSONValue]: unrecognized selector sent to instance 0x43897f0'
Everything else in the static library continues to function properly.
Apparently, categories and static libraries don't work unless you sprinkle the magic dust on the linker flag. According to a Technical Q&A, you have to add the -ObjC linker flag to the main project (not the library, as stated in the Q&A).