Continuity mistake: When Lisa is first delivering her newspaper, Red Dress Press, she unties a pile and the strings are untied. Then the shot changes and the pile has been done up instantly. (00:05:55)
Continuity mistake: When Lisa is interviewing people for her newspaper articles, the order is Nelson, Ralph and then the two purple twins, Sherry and Terrie. But after Ralph has been seen, it is not the twins but a totally different person, Lewis. (00:07:30)
Continuity mistake: Throughout the whole of this episode, the picture keeps on changing on Mr. Burns' wall. Sometimes even in the same scene.
Continuity mistake: Throughout the whole of this episode, the clock on Mr. Burns' desk keeps appearing and disappearing.
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