Audio problem: When the family are about to enter the Knowledgeium Homer says 'Will there be beer?' but his lips don't move.
Audio problem: At the Penitentiary gates when Ralph says 'Daddy's key went in there,' his lips don't move.
Audio problem: When Bart is reading Marge's police file he says 'Oh, Mum's only been in twice.' But the lip sync is a bit out because it does not match the words he says.
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