Audio problem: When Bart is attempting to skate over Homer's car with his friends watching, Milhouse says 'You're crazy Bart.' but his mouth never moves. (00:10:55)
Audio problem: At the gorge, Nelson says, "Where is he? I thought he said noon!" But Milhouse's mouth is also accidentally lip synced to Nelson's dialogue.
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