Itchy and Scratchy: The Movie - S4-E6
Revealing mistake: When Lisa is trying to explain to Homer about Bart seeing the movie, one of the pearls on her necklace keeps disappearing and reappearing. (00:18:20)
Itchy and Scratchy: The Movie - S4-E6
Revealing mistake: When Marge says she's sure Bart doesn't mean to be bad, her necklace flashes through her neck. (00:03:40)
Itchy and Scratchy: The Movie - S4-E6
Revealing mistake: When Nelson blinks right before Milhouse says "No one who saw the movie would say that", Nelson's right eyelid is white for some reason. (00:18:20)
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