The Man in the Blue Flannel Pants - S23-E7
Audio problem: When the woman on the DVD Bart is watching says "Are you sure you're Martin Scorsese?" her mouth is completely out-of-sync with her voice.
The Man in the Blue Flannel Pants - S23-E7
Audio problem: In the rafting scene, while Marge says "And maybe when we get home, we can ride down something else?" part of her dialogue does not match her mouth movements.
The Man in the Blue Flannel Pants - S23-E7
Continuity mistake: While Krusty is talking to his merchandising executives, after Krusty says "Get to the point, unless you want a face full of clown chunks!" part of his collar turns green.
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