Continuity mistake: The shaving cream on Chief Wiggum's face disappears when Bart tries to confess about the phony kidnapping.
Continuity mistake: Chief Wiggum's badge is shown to disappear under his fat in one scene, but in the following shot of him the badge is completely visible.
Continuity mistake: When Bart goes up to his treehouse to write the kidnap letter, he throws his Murder 4 Life jumper out the treehouse entrance. Later, when Lisa finds the jumper, it is now on a tree branch as if he threw it out of one of the treehouse windows.
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