Children of a Lesser Clod - S12-E20
Continuity mistake: When Ned Flanders is asking Homer whether or not he is running a daycare centre, there is a radio behind him. The aerial on it changes position in subsequent shots.
Children of a Lesser Clod - S12-E20
Continuity mistake: When Bart and Lisa are trying to win back Homer's affection they are sitting on the top of the stairs and push the skateboard with the present on it down the stairs. In the next shot it rolls into Homer's legs yet it would be impossible as he was standing in the main living room which is to the right of the stairs. Pushing the present down the stairs it would go straight into The Front door.
Children of a Lesser Clod - S12-E20
Audio problem: When the moon says, "You go, Homer,'" the words dont match what the moon says.
Continuity mistake: When Lisa comes and starts giving out apples, Moe is seen taking one of them, but some seconds later, he is seen under Homer's coat with a piece of Homer on a fork.
Tree House of Horror XII - S13-E1
Continuity mistake: In the opening sequence, when the coffins shoot out of the grave, one of them changes colour between shots, when it hits the pillar. (00:00:25)
Tree House of Horror XII - S13-E1
Continuity mistake: In 'Hex and the City,' as they walk through Ethnictown, Marge has a handbag over her shoulder. But in the closeup when she says "Oh, listen.", the bag has gone. (00:01:15)
Tree House of Horror XII - S13-E1
Continuity mistake: At the end of 'Hex and the City,' there is a shot of the wedding cake, then it pans across the people at the wedding, and you can see Homer with his white shirt on. But then when it goes to a closeup, he suddenly has a blue suit and tie on. (00:06:45 - 00:07:20)
Tree House of Horror XII - S13-E1
Continuity mistake: In 'House of Whacks,' Homer swings an axe at the water softener, making three large dents. The shot changes and one of the dents near the bottom vanishes. (00:13:30)
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)
Tree House of Horror XII - S13-E1
Continuity mistake: In 'Wiz Kids,' we see Ralph tied up in Lord Montymort's lair. Between a wide shot and closeup, when he says "Dying tickles," his cloak goes from undone to done up. (00:17:05)
Tree House of Horror XII - S13-E1
Continuity mistake: At the show the school puts on in "Wiz Kids", when the Montymort Dragon is first seen in the cage, the cage has 6 bars at The Front. When Lisa says "Release The Dragon", the cage has 4 bars. When the cage door is being raised, it has 6 bars again. (00:18:45)
Tree House of Horror XII - S13-E1
Continuity mistake: In "Wiz Kids", Bart stabs Montymort in his shin which is open (i.e. not covered by what he's wearing). Only a little blood pours out. Then Monty raises his shin into shot and there is not only a puncture in the leg of his clothing, but the wound is covered in blood. When he falls, and Slithers eats him moments later, there is no damage at all. (00:19:55)
Tree House of Horror XII - S13-E1
Continuity mistake: At the end of 'Wiz Kids', when Lord Montymort is defeated, he falls to the floor, breaking all the floorboards. Then when Lisa takes off the headset, all the floor is intact. In the next shot, the floorboards are all broken again. (00:20:05)
Tree House of Horror XII - S13-E1
Continuity mistake: In the end credits when the stars exit the caravan, there is a wooden step under the door of the caravan. However, when they climb into Pierce's car and drive off, the wooden step is gone. (00:20:55)
Tree House of Horror XII - S13-E1
Continuity mistake: In the end title sequence, the frogman thing vomits and leaves a stain on the parking lot. When Pierce Brosnan drives away in his car, there is no stain. (00:21:05)
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.
Tree House of Horror XII - S13-E1
Continuity mistake: In 'Hex and the City', Marge and Homer swap places at the wedding.
Tree House of Horror XII - S13-E1
Continuity mistake: When the family finds Homer, he has an hole behind his head and you can see his brain, but it's not there later.
Tree House of Horror XII - S13-E1
Continuity mistake: In the classroom scene in the school Nelson is beating on Milhouse and leaves his head black, but when they go to get graded Milhouse is in front of Lisa (not Nelson) and his head is normal.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Wiz Kids", when Milhouse is performing his trick at Springwart's Magic Recital, the trick goes wrong and he runs off naked, leaving his cloak on the floor. Later on in the scene, the cloak disappears.
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