Three Men and a Comic Book - S2-E21
Trivia: Bart is surprised to learn that the actor who played Fallout Boy hadn't died in Vietnam. This is a reference to when both AP and UPI erroneously reported that Jerry Mathers of 'Leave it to Beaver' had died in Vietnam.
Brian Katcher
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