Sideshow Bob's Last Gleaming - S7-E9
Chief Wiggum: Hey, where is Sideshow Bob and that guy who, uh, eats people and takes their faces?
Guy Who Eats People and Takes Their Faces: I'm right here, Chief.
Chief WIggum: Oh. Then where's Sideshow Bob?
Prisoner: Eh, he ran off.
Chief Wiggum: Oh great! Well if anyone asks, uh, I beat him to death. Okay?
Officer Lou: Right.
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