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
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
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.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Homer attacks the tank, he puts the axe in the right hand side of the tank, but in the next shot the axe is now in the middle of it.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when the tools are attacking Homer, you can see one of the tools puts shaving foam on his face. But when he is walking down the stairs, the foam has gone.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Homer is sitting on the settee, you can see his beer glass is empty, even though a few seconds ago it was nearly full.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when the dishes are cleaned, you can see by Marge's plate there is a spoon and a fork. In the next shot, there is now only a spoon, which disappears in the next shot.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Marge has finished eating her meal, you can see a spoon on her plate. But when she is seen on the camera, the spoon has gone.
Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Marge opens the door to the robotic salesman, you can see there is a green mat under the door. But when the salesman puts his foot between the door, the mat is missing, and in next shot the mat has re-appeared.
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