The Blue and the Gray - S22-E13
Audio problem: When Homer says "Drive my kids to school?!" his mouth is out of sync with his voice.
Audio problem: When Marge says "Now that it's empty, who's going to buy their house? What if someone moves in with two Barts? Or four teenage Barts?" her mouth movements don't match most of her dialogue.
Don't Fear The Roofer - S16-E16
Audio problem: When Marge says "You're drooling on my goodwill pile", her mouth doesn't match her voice.
A Star is Born-Again - S14-E13
Audio problem: When Homer tells Ned "Flanders, I'm always interested in talking about free milk. And if there's cookies involved, so much the better", his mouth is not in sync with his voice.
Strong Arms of the Ma - S14-E9
Audio problem: When the woman who smashes the piano says "support the arts", her mouth is open, but it doesn't move.
Half-Decent Proposal - S13-E10
Audio problem: The lady singing at the prom doesn't move her lips when she sings, her mouth just stays open.
Audio problem: When Homer says "He sees a thousand times worse on that animal channel", his mouth is out of sync when he says "channel."
New Kids on the Blecch - S12-E14
Audio problem: When the VJ on the TV says "That was the latest ad from Stridex pads. Medicated", her mouth is terribly out of sync with her voice.
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.
Brother's Little Helper - S11-E2
Audio problem: Homer's mouth is out of sync when he's naming celebrities who use drugs.
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.
Audio problem: In the U2 scene during the end credits, when The Edge says "oh, here we go" his mouth is not in sync with his voice.
Audio problem: Marge, Bart and Lisa are still heard "oohing" at the plumbing supplies store even after they close their mouths.
Audio problem: When the car drives into the cornfield, Martin is heard screaming, even though he is still asleep.
Scenes From a Class Struggle in Springfield - S7-E14
Audio problem: Homer's mouth is completely out of sync when he says "our savings?"
Three Men and a Comic Book - S2-E21
Audio problem: When Homer offers to take the family to Krusty Burger, Lisa is heard cheering, even though her mouth is closed.
Audio problem: When Bart says "I'm going to find my dog" his mouth does not match what he is saying.
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Audio problem: In "Clown Without Pity" when Homer yells "The doll is trying to kill me, and the toaster's been laughing at me!" his mouth doesn't move. (00:06:45)
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