Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: When the last wish is granted by Homer, when the index finger moves, you can see the refrigerator to the right of the hand. Homer was standing with his right arm facing the fridge. (00:06:55)
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
Continuity mistake: At the very end of Part 1, Homer changes position between shots, and when Flanders wants a huge castle, it suddenly changes back to his normal house. (00:08:30)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: When Bart gets on the bus, we see one of the twins sitting near The Front of the bus. In the next shot, we see both twins sitting at the back. The shot after that, we see the twin near The Front again, running to the back. (00:10:30)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: At school, there are two rows to the right of where Bart is sitting. This keeps changing from two rows to one row during the entire scene. (00:11:00 - 00:12:15)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: While Bart is in school, Sherri and Terri are making him comfortable. One of the twins vanishes for one shot, and their skin color keeps changing from white to yellow. (00:11:05 - 00:12:15)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: When Homer gets turned into a jack-in-the-box, we don't hear the chair he was just holding crash onto the floor. Bart doesn't make it vanish, as we see it on the floor when Homer is turned into a jack-in-the-box. (00:13:20)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: In Dr. Marvin Monroe's office, in the first shot, we can see a garbage can beside his desk. It disappears for the rest of the scene. The desk also moves forward. (00:13:35)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Other mistake: As Mr. Burns' fireplace rotates, the items on the top should have fallen off, but they didn't. They also disappear/reappear between frames. (00:16:10)
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: During the 'Next week, on the Simpsons' clip, Maggie changes where she sits at the table. Lisa also changes where she sits, as you can see in the background. (00:21:45)
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
Continuity mistake: When Jimbo and Kearney take Homer's candy, it is obviously night time. However if you watch the outside as Homer closes the door before he says "Lousy punks" it shows a daytime sky.
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.
Tree House of Horror II: A Simpsons Halloween - S3-E7
Continuity mistake: When Mr. Burns is telling the robot Homer that he is his daddy, Burns is to the right of Homer, but after the POV of Burns' face (he is still oriented right of Homer) by Homer, Burns is now to the left of Homer.
Tree House of Horror II: A Simpsons Halloween - S3-E7
Other mistake: As Kang and Kodos fly away from Earth in their spaceship, you can see that the Earth has been coloured wrong. The continents are blue, and the sea is green.
Lisa's Pony - S3-E8
Continuity mistake: Homer writes '4 1/2 Reed' on his left shoe. When the owner of the music store asks him what Lisa plays, he is reading off his right shoe. (00:02:00 - 00:04:40)
Lisa's Pony - S3-E8
Continuity mistake: The people sitting behind Marge during the talent show keep changing/disappearing. (00:02:10 - 00:06:00)
Lisa's Pony - S3-E8
Continuity mistake: The number of knives that Sherri is holding changes between long and close shots at the talent show. (00:04:50)
Lisa's Pony - S3-E8
Continuity mistake: Homer's digital watch disappears when he enters the auditorium. It also vanishes in the first shot of Homer in the music store. (00:05:05 - 00:06:25)
Lisa's Pony - S3-E8
Continuity mistake: During Lisa's performance, In one shot of the audience we see, from left to right: Ned Flanders, Maude Flanders and some lady. The rest of the talent show showed us that Jasper was sitting where the lady was. (00:05:45)
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.
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