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.
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