Audio problem: When Homer and Marge are driving to the hospital, Marge says "Let's just say that Bart did it". But her mouth says something else when she says "Bart".
Audio problem: When Bart is going through his keys, he says "Your house", and his mouth doesn't match what he is saying.
Audio problem: When Marge says "We've gotta get to the hospital, Homer" her mouth doesn't move.
Audio problem: When Lisa says "Oh no, somebody took my bike", she is mouthing something else.
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