Sideshow Bob's Last Gleaming - S7-E9
Continuity mistake: When Maggie (sitting on Marge) appears on "Tyrano-Vision," Homer waves his hand in front of her. However, we can see that he isn't leaning over too far, and in the next shot it is clear that both Bart and Lisa are sitting between Homer and Marge, so there is no way he could have reached over to her in the first place.
Sideshow Bob's Last Gleaming - S7-E9
Continuity mistake: When Bart and Lisa first enter the Duff Blimp, Bart doesn't have his back-pack on. However, he is suddenly wearing it after the bomb has gone off.
Sideshow Bob's Last Gleaming - S7-E9
Continuity mistake: When Homer drives under the plane to save Bart from Sideshow Bob, you can see he drives under the plane twice.
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.
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