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 intended to tell a normal joke, but his mouth, which appeared to have its own will, said an alternative punchline, concluding in a deadly joke and forcing Gary to record the joke on tape and listen to it repeatedly until he became accustomed to the joke.