Revealing mistake: On the original DVD version, when Bart says to Lisa "I didn't want you to wreck your life" the screen is cut off on the right side slightly. (00:20:25)
Continuity mistake: When Bart is digging his hole, Homer goes over and starts talking to him, and you can see a spade standing up outside of Bart's hole. The spade wasn't there in the previous shot of Bart digging. (00:02:05)
Continuity mistake: This episode rewrites the entire backstory of Marge and Homer. For starters, Lisa and Bart were both not born yet, despite the fact that in the episode "Lisa's First Word," the '84 Summer Olympic games were being broadcast, and Bart and Lisa were both there. In this episode, Homer and Marge were just dating, but again in "Lisa's First Word," they were married and living in their Evergreen Terrace house. Additionally, Maggie was born in the episode "And Maggie Makes Three." Homer also lost all his hair in that episode, yet in this episode in the early 90's he still has a full head of hair.
Continuity mistake: When Lisa finds the doorbell Marge wanted and she plays it, it is gold, but when Marge picks it up it is grey-ish.
Continuity mistake: As Marge is feeding Maggie a grapefruit, Bart takes out his bongo drums and plays them. Watch as Bart plays - the shot zooms in from the back of Bart towards Maggie, and you can't see Marge in the scene sitting close to Maggie; the next shot shows her though.
The Two Mrs. Nahasapeemapetilons - S9-E7
Continuity mistake: The amount of liquified chips in the bag at the home changes depending on the shot.
Last Exit to Springfield - S4-E17
Continuity mistake: When Mr Burns is telling Homer to resign, he has glasses on in the first shot in the scene. However his glasses are on his desk the next shot onwards.
Homer vs. Patty and Selma - S6-E17
Other mistake: When Homer burns a dollar bill, he turns his cigar around 180 degrees. If you look carefully, the smoke soon comes from the different side than it did earlier.
Simpsons Bible Stories - S10-E18
Continuity mistake: Homer as Solomon cuts the pie into a roughly 1/3 and a 2/3 piece, but when he picks the pieces up, it's more or less halved.
Tree House of Horror V - S6-E6
Continuity mistake: At the end of 'The Shining, ' the family sits in the snow, Homer holding Willie's portable TV. Later, when they're frozen, Homer is holding nothing, although the TV's snow-topped antenna is still there. Additionally, Homer's left shoulder is missing. This is all due to the lack of correct colorization.
They Saved Lisa's Brain - S10-E22
Character mistake: The plural form of ignoramus is ignoramuses, not ignorami. The word "ignoramus," a Latin verb-form meaning "we do not know," has no Latin plural-noun form. Therefore, when used as an English noun it correctly pluralizes as "ignoramuses." Someone as smart as Lisa should know this.
White Christmas Blues - S25-E8
Continuity mistake: While Mayor Quimby is giving his speech, Moe is in two places at once. He is simultaneously standing next to Mayor Quimby and amongst the crowd of people.
Continuity mistake: When Mr. Burns is confronting Homer about his missing slave, Homer's donut is half eaten. During the next scene the donut has a smaller bite taken out of it. (00:14:00)
Character mistake: Lisa states that Pierre and Marie Curie both died of radiation poisoning. This is incorrect, as only Marie died of radiation poisoning while her husband Pierre was killed by a horse and cart. (00:08:30)
Continuity mistake: When the family are on the conveyer belt at the beginning the family are travelling along. In the next shot (the close up) they are travelling alot slower.
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)
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: The card Bart has written for Milhouse is blank inside. However when Lisa looks at the card, the pun of the joke (on The Front of the card) is now there. (00:01:15)
Homer's Barbershop Quartet - S5-E1
Character mistake: When Marge is making a model of Homer for Bart and Lisa, she makes him bald with his hair round the back of his head as we all know it, but at this point in the story Homer was not yet bald. (00:18:15)
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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