Treehouse of Horror XIII - S14-E1
Zombie Billy the Kid: Now I'd like you to meet the Hole in the Ground gang: Frank and Jesse James. The Sundance Kid.
Comic Book Guy: What happened to Butch Cassidy?
Zombie Sundance Kid: "What happened to Butch Cassidy?" We're not joined at the hip, you know.
Zombie Billy the Kid: And the most evil German of all time: Kaiser Wilhelm.
[Everyone is confused]
Zombie Frank James: He ain't a cowboy!
Zombie Kaiser Wilhelm: Sure I am. Yippee. Whippee. Whippee.
Zombie Frank James: Okay, he's in.
Treehouse of Horror XIII - S14-E1
Zombie Billy the Kid: [to Homer] Play us some piani. [Homer begins playing "Fur Elise" and Billy the Kid fires his guns in response] That's piano, I said piani! [Homer begins playing some honky tonk].
Answer: I'm not sure if this is the one you're thinking of, but an episode of "Ray Bradbury Theatre," called "A Sound of Thunder," dealt with a similar matter: a group of hunters travel back in time to hunt dinosaurs, only to find things have changed when they get back because someone stepped on a butterfly.
Xofer