Krusty Gets Kancelled - S4-E22
Other mistake: When the Red Hot Chili Peppers are performing on Krusty's TV special, bassist Flea is playing a standard electric guitar.
Continuity mistake: When Homer ignores Lily, she gets upset then runs out of Moe's, and in the interior shot the door is hinged at the left, but in the next exterior shot that door is hinged on the opposite side, while Lily is crying outside. (00:09:35)
Continuity mistake: When Marge walks out of her aunt's house onto the porch, the screen door's handle is at the right side, but in the following shots the screen door opens the opposite way. (00:10:05)
Continuity mistake: While Lily continues to "light up all the men's lives, except Homer's," when she walks into the power plant with picnic lunches, Homer is wearing a solid peach colored tie, but in the next shot it's a striped peach and pink tie. (00:11:00)
Continuity mistake: When Homer takes Lily home after her date with Burns, as Homer pulls up to her house, the doorknob is on the left side of its front door, but when Lily enters the house, the doorknob is on the opposite side of the door. (00:16:20)
Continuity mistake: At the end, when Lily is back at a pub in England and meets Homer's doppelganger, his single shirt pocket and button are at his right side, but as he and Lily leave the pub arm in arm his shirt pocket and button are on the opposite side. (00:20:20)
Homer Alone - S3-E15
Continuity mistake: When Homer is singing to Maggie, he is standing by the bed. But in the next shot, Homer disappears and reappears.
Waverly Hills 9-0-2-1-D'oh - S20-E19
Revealing mistake: In all the intro scenes from season 2 to season 20, episode 9, in the wideshot where Bart skateboards out of the school, if you look closely, part of the sky can be seen on the ground. (This mistake may only apply to the original format).
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)
Continuity mistake: When Comic Book Guy turns off the water hydrant, the two boys in purple and blue trunks change position between shots.
Character mistake: When Bart fakes being sick, on his way to the infirmary he passes the door with a sign that reads "maintanence" but the proper spelling is "maintenance", and when Bart reaches the infirmary, the sign on the door reads "ineirmary." (00:07:30)
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)
Continuity mistake: Seymour's time as a POW in Vietnam changes throughout the show. One episode he mentions having been one for a year and a half. Another one he says it was two years. A third one he says it was three years.
Continuity mistake: When Tony Hawk and Homer compete against each other on the skateboards, Bart is on Tony's starting side. When Homer knocks Hawk off his skateboard, Bart is now on the side that Homer started on.
Clown v. Board of Education - S34-E21
Continuity mistake: Fat Tony burns Krusty's tassel on his graduation cap. This causes Krusty to put it out and move it behind his head. In the next shot, it jumps back to the right side of the hat.
Character mistake: When Hugh proposes to Lisa using the electronic signage, he spells the word "honor" using the American English spelling. Hugh should have used "honour" as this part of the show was set in England, and Hugh is from England.
Other mistake: During the Planet of the Apes musical final number, as the chorus line dances on stage from left to center with the Statue of Liberty in the background; however, parts of the Statue (the pink background between arm and book) cut off the dancer's head as if it is in the foreground. (00:12:19)
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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