There's a list in the gfa manual. When gfa gives you a bogus error message you've done something so terribly bad that it sent the cpu off in to la la land. Probably executing garbage as code somewhere.
I assume you called some asm routine or POKEd some address that corrupted the running program itself. The only way to mess up the stack is calling some badly written asm. There are other registers you must not destroy or gfa freaks out. It's explained in the manual, see the chapter on C:() or CALL or RCALL.