Continuity mistake: In the scene where Homer is going down the escalator, in the Costington's Centre, after he gets on two men follow him, then a woman, yet when the shot changes, it's a man then a woman then the other man. (00:00:45)
Continuity mistake: In the scene where Bart is in the Court House, when we first see everyone, on the Judges table there is no gavel, yet when the shot changes, a gavel appears. (00:05:20)
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