Gary had meant to say the normal joke but his mouth, which seemed to have a "mind of its own," said a different punchline, ending in a killer joke and forcing Gary to play the joke on a tape and listen to it many times until he got used to the joke.
Gary had planned to tell a standard joke, but his mouth, behaving independently, said a different punchline that ended with a deadly joke. This required Gary to record the joke onto a tape and repeatedly listen to it until he was familiar with the punchline.