I Can Crack Your App With Just A Shell (And How To Stop Me)
The most stealth cracking countermeasure I ever witnessed was the application would XOR some of its UI messages with the hash sig of the application binary file, so if you edited the application binary file directly the crack seemed to work just fine ... but then the application would gradually go insane. The cracker who finally posted a working crack was  impressed with how simple and devious the countermeasures were.