Audio problem: After the donut delivery guy asks Carl why all the donuts are piling up, Carl responds with Lenny's voice.
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)
There's Something About Marrying - S16-E10
Audio problem: Homer is strangling Bart and tells Lisa to hold his spot. You can hear Lisa say, "Why you little," and you can hear Bart choking, but neither of their mouths is moving.
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)
Tree House of Horror XII - S13-E1
Audio problem: In 'Wiz Kids,' when Edna asks Harry Potter if he is chewing gum, he replies "No ma'am, it's brimstone," however it's clear that a different line was originally intended. (00:15:25)
The Two Mrs. Nahasapeemapetilons - S9-E7
Audio problem: When Homer is running for the food, after racing in the wheelchair with Jasper, he says 'Eat my dust' but his lips don't move for the 'my dust' part.
Audio problem: Just after Mr Smithers says "...so that we have the energy to from your favourite video game to yummy cotton candy machines", all the children start clapping but the sound of clapping starts BEFORE they start clapping. (00:05:05)
Audio problem: Both Lenny and Carl's voices are wrong. At one point Carl's voice is higher pitched, and Lenny keeps speaking like Moe, or like a voice we haven't heard before.
Audio problem: After Homer and Marge steal Gil's balloon, they fly towards clothes, Marge says "Clothes! We're saved!", but her mouth dosen't move.
Audio problem: At the theatre, when the cook talks to Mark Hamill off-stage he sounds exactly like Moe the bartender.
Tree House of Horror XII - S13-E1
Audio problem: In Reaper Madness; the scene where Homer destroys the Grim Reaper with a bowling ball, Bart is pinned on the wall by the Reaper's scythe. The scythe disappears and Bart says "Cool!" but his mouth just opens up and doesn't match the word.
Mr. Spritz Goes to Washington - S14-E14
Audio problem: At the end of the episode, when the guy who helped the Simpsons pass the bill says, "I just passed a bill in three hours. Quit complaining," his mouth movements don't match the words.
Homer Alone - S3-E15
Audio problem: When Lisa screams "Mom" on the way to school, she just leaves her mouth open and doesn't close it. It should sound like "Maaaaaa" if she keeps her mouth open. (00:03:20)
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.
Simpsons Spin-off Showcase - S8-E24
Audio problem: In Moe's spinoff, when we first hear Grampa's voice, you can see the love tester in the background but the love meter lights do not light up when he speaks.
Audio problem: When Bart says "Krusty Hotline" his mouth doesn't move.
Audio problem: When Bart calls Milhouse to get the answers to the history test from him, Milhouses voice sounds a lot different from his usual voice. It sounds more like Mrs. Krapabbel's voice. (00:08:30)
Tree House of Horror V - S6-E6
Audio problem: In 'Time and Punishment,' Lisa yells "Dad your hand is jammed in the toaster!" and we then hear her screaming in the following shot, however her mouth is closed. (00:08:55)
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)
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)
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