Tree House of Horror IX - S10-E4
Audio problem: At the end when Bart says 'Don't forget Ken Star' his mouth doesn't move.
Audio problem: In the beginning, we are at the power plant. We find out that Burns has packed his own lunch, and one thing he packed is a bullion cube. His mouth doesn't move right when he says "Bullion cube".
Audio problem: Superintendent Chalmers' mouth doesn't move when he says "Idiot."
Audio problem: When Homer says "According to my driver's license, I weigh 140 pounds" his mouth is not in sync with his voice.
Audio problem: At the theatre, when the cook talks to Mark Hamill off-stage he sounds exactly like Moe the bartender.
Audio problem: At the end, where Homer is carrying Mark Hamill out of the theater and Mark says, "Shoot, I forgot my Light saber", his mouth doesn't match what he says, and it keeps moving when he's quiet.
Audio problem: At the end where Homer is carrying Mark Hamill out of the theatre, Mark's lips don't match with what he's saying.
Wild Barts Can't Be Broken - S10-E11
Audio problem: When Lisa is broadcasting secrets about Springfield residents in a British accent, her lips don't match her mouth at all.
Audio problem: When the radio is playing "Wannabe", the song stops but Homer doesn't touch the radio.
The Old Man and the C Student - S10-E20
Audio problem: At the choir at the start, Lisa's lips don't move when she says "oh no."
The Old Man and the C Student - S10-E20
Audio problem: At the end of the episode, Grampa's mouth doesn't move when he sings.
Thirty Minutes Over Tokyo - S10-E23
Audio problem: In America Town, when Bart says "These prices suck. 10,000 Yen for coleslaw?", his mouth doesn't match what he is saying.
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