Factual error: You wouldn't hear a dial tone on one side of the phone line if the phone on the other side was cut from the receiver. You wouldn't hear anything.
Continuity mistake: During a shot of everyone at church, Carl is seen with a red sweater, after Homer says "Dammnit!", Carl's sweater turns blue.
Other mistake: When Ned Flanders is calling Marge and Reverend Lovejoy from the Leftorium, he says that the three hooligans are in the shop on their motorbikes. But a few minutes before when Ned goes outside the shop to tell the boys to clear off, their bikes are nowhere in sight.
Continuity mistake: In the cafeteria at the power plant, in the wide shot the clock on the wall is on 9.00, but closer up it's showing 10.35.
Continuity mistake: The hole in the wall made by the acid changes shape and size in several scenes.
Continuity mistake: Lenny says that Homer got his job by showing up when the nuclear plant was opened. In the episode "I Married Marge" (Season 4), the plant is already open when Homer interviews for a position, doesn't get it, then works at Gulp 'N' Blow.
Continuity mistake: After Milhouse explains to Bart how the warehouse collapsed, he appears to be bald for some reason.
Revealing mistake: When Lenny is talking about Homer not needing a degree to work at the power plant, Lenny's five o'clock shadow briefly detaches from his face.
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.
Simpsons Spin-off Showcase - S8-E24
Continuity mistake: In Moe's spin-off, just before Barney leaves the bar, his beer glass vanishes.
Simpsons Spin-off Showcase - S8-E24
Continuity mistake: In the beaver sketch, "Lisa" is holding Maggie in front of her to show everyone her first tooth. In the next shot, Maggie is instantly to her side.
The Secret War of Lisa Simpson - S8-E25
Other mistake: When Lisa looks down from the platform before crossing the obstacle course, you can see her feet, but not the platform she stands on. (00:19:04)
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: When Lisa and Bart are practising for the 'Eliminator', Lisa doesn't have a rope round her waist, but when she falls she does.
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: When Lisa climbs to the top of the Eliminator, there is a platform for her to stand on. Strangely in the next shot Lisa is standing on nothing so she appears to be standing in mid-air.
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: The vehicle they are driving changes form when they drop off Bart at military school to when they drive home. They leave with Homer's purple car (obvious from the crumpled headlight) and as they leave the military school it's Marge's orange car (obvious from the ski racks and lack of a trunk). It's clear from the entire scene that this is all one trip.
The Secret War of Lisa Simpson - S8-E25
Factual error: When Bart smashes all the glass in Springfield(as a result from the sound wave he made by speaking through the mega phones)the car windows get smashed as well. But when we see Chief Wiggum and Homer's car, the glass is still intact.
The Secret War of Lisa Simpson - S8-E25
Continuity mistake: When Lisa has just been saved from falling, from the Eliminator at night, three boys approach. The two boys on the right swap positions between shots.
The City of New York vs. Homer Simpson - S9-E1
Continuity mistake: Just before Homer tries biting off the boot, he is wearing a watch. But from then on it disappears off his arm until he is trying to figure out how many hours it is from 9-5.
The City of New York vs. Homer Simpson - S9-E1
Factual error: The windows of the World Trade Center couldn't open and the Twin Towers weren't in front of each other as depicted in this episode. They were at an angle.
The City of New York vs. Homer Simpson - S9-E1
Continuity mistake: When the camera pans down the Trade Center, there is no washing line there. But a few shots later, a washing line appears.
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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