Much Apu About Nothing - S7-E23
Continuity mistake: When Willy is threatening the children and Skinner it shows that Lisa has her hands on her sides and she is holding nothing. In the following shots, she has a blue book held against her chest.
Much Apu About Nothing - S7-E23
Continuity mistake: When the mob return to Quimby his assistant's clipboard changes colour between shots.It goes from light brown to burgundy then back to light brown again.
Much Apu About Nothing - S7-E23
Revealing mistake: There are two Ralphs when Willie says: "You want to pick on immigrants?" (00:06:35)
Much Apu About Nothing - S7-E23
Audio problem: When they shout 'No on 24!', none of the people move their lips. (00:21:03)
Much Apu About Nothing - S7-E23
Continuity mistake: During the party for Apu, Dr. Nick is first seen not wearing a coat, then he has his usual white coat, then he has a brown coat, and then the white coat again.





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