Audio problem: When Bart says "Krusty Hotline" his mouth doesn't move.
Audio problem: When Homer is in the asylum talking in his sleep, his mouth stays open when he's talking.
Audio problem: When Michael and Bart are playing a song to Lisa, Bart uses her bin as a drum. Sometimes you hear tapping, but Bart doesn't touch the bin.
Mr. Lisa Goes to Washington - S3-E2
Audio problem: When Lisa is reading her speech for the essay contest, what she says does not match her mouth movements. (00:06:30)
Mr. Lisa Goes to Washington - S3-E2
Audio problem: When the lady is asking questions to Homer before Lisa wins the trip to Washington, what she says doesn't match her mouth. (00:07:20)
Audio problem: During Marge and Homer's argument at the beginning, when Homer says that Ned Flanders is a jerk, his mouth doesn't match what he's saying. (00:01:55)
Audio problem: When Ned calls Homer and says "over here" his mouth doesn't move. (00:17:30)
Audio problem: When Otto says that he was on his way to Mexico, his mouth doesn't move in the interior shot of the bus. (00:04:20)
Audio problem: After Barney falls over in his chair, you can hear him say, "Hmm, a pretzel." His mouth doesn't move. (00:21:15)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Audio problem: When the guy who Homer buys the monkey's paw from tells him that he'll be sorry, his mouth doesn't move. (00:03:10)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Audio problem: Right after Flanders makes his first wish, when Moe chases the alien, Moe is talking, but his mouth doesn't move. (00:08:00)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Audio problem: In the opening when Marge is warning us about how bad this years episode is, her mouth barely matches what she says.
Tree House of Horror II: A Simpsons Halloween - S3-E7
Audio problem: When Kang and Kodos land in Springfield and hit Mayor Quimby with a club, one of them says something but his mouth is not moving.
Lisa's Pony - S3-E8
Audio problem: When Homer is crying after the tape is finished, what he says doesn't match his mouth. (00:07:25)
Lisa's Pony - S3-E8
Audio problem: When Lisa rides past on her pony, Ralph is sitting on the fence with another boy who says, 'She really tamed that horse', Ralph says, 'Yes but who can tame her?' However he speaks with a deeper voice as opposed to his higher squeaky voice, and sounds exactly like Nelson Muntz. (00:15:55)
Audio problem: When Homer sulks about not knowing anything about Bart, his first words of speech doesn't match his mouth. (00:08:20)
Audio problem: When Krusty arrives at the bar, Bernard says "Hey Krusty" but his mouth doesn't move. (00:11:00)
Audio problem: When Moe says "I'll talk to you tomorrow" to Homer, his mouth doesn't move. (00:15:15)
Burns Verkaufen der Kraftwerk - S3-E11
Audio problem: When the drunks find out they're going to Hawaii, one of them says "Maybe I'll marry Elizabeth Taylor." If you look at their mouths, though, they don't move right. (00:09:50)
Burns Verkaufen der Kraftwerk - S3-E11
Audio problem: When Horst is checking the power plant, he sees some radioactive liquid pouring out of the chimney, he says "That shouldn't be" but his mouth doesn't move when he says "be". When he spots the raccoons he speaks but his mouth does not move at all. And at the end when Mr Burns is mocking the German men, Horst says "Burns, stop it." but his mouth doesn't move. (00:15:30)
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