Continuity mistake: When Homer comes into the kitchen to ask what 1 thing Lisa would want most, the milk carton and glass disappears from the table for a few shots. (00:07:10)
Continuity mistake: The time on the clock when Homer comes into the kitchen keeps jumping forward in massive leaps. (00:07:30)
Continuity mistake: When Homer and Marge are talking about Homer seeing Lisa through a fathers eyes, the clock on the wall simply disappears. (00:08:00)
Continuity mistake: When Lisa says she will do the pageant, her clock goes from 8:05 to 7:55. (00:08:30)
Continuity mistake: When Krusty appears on stage he has a microphone that vanishes from his hands when he claps at the end of the song. (00:13:10)
Continuity mistake: When it is announced that Amber is the winner of the contest, and Lisa is runner-up, the stands in the back of the stage are empty. However, when Lisa walks off, all the contestants are back on the stands. (00:15:00)
Revealing mistake: Krusty's nose flashes white for a split second during his song for the winner of the Little Miss Springfield pageant. (00:15:06)
Continuity mistake: When Krusty the Clown arrives at the beauty pageant his tie is done up. When he goes on stage it is undone.
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Continuity mistake: When Bart gets the Krusty doll, he pulls the pull string from the back of the doll. When the doll has finished talking, the pull ring has disappeared. (00:04:05)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Continuity mistake: When Homer finds out the doll is trying to kill him, the popcorn moves from the sofa onto his lap. (00:04:40)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Audio problem: In "Clown Without Pity" when Homer is singing in the bath, his mouth doesn't match what he is singing. (00:05:20)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Audio problem: In "Clown Without Pity" when Homer yells "The doll is trying to kill me, and the toaster's been laughing at me!" his mouth doesn't move. (00:06:45)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Deliberate mistake: In "Clown Without Pity", when the Krusty doll accidentally knocks the head off of Lisa's Malibu Stacy (her doll), Krusty puts it back on but you don't see a crack where the head came off. (00:07:40)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Audio problem: In King Homer, after Burns and the guards on the island rush through the doors, you hear Burns say "Shoot him in the groin and belly!" But his mouth doesn't move. (00:10:30)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Continuity mistake: In King Homer, When Burns is telling the reporters what the ape will be doing, the crowd in the background are devoid of any facial features. (00:11:40)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Plot hole: At the theatre in the "King Homer" bit, the audience are treated to a traditional showgirl spectacle. But the girls have no curtain behind them, and so the audience should have seen Homer the ape, but they act completely surprised a minute later when Homer comes on. (00:11:40)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Continuity mistake: When Homer is escorting his family out of the house to escape the Zombies, he leaves the door wide open, but in the very next shot it is shut. (00:18:50)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Deliberate mistake: In "Dial Z For Zombies", Homer is shooting zombies with the shotgun at the school. When he shoots the first two he just shoots them twice. Every other zombie he shoots requires him to slide the gun. It doesn't when he shoots the first couple of zombies. (00:19:50)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Audio problem: When Lisa is turned into a snail in the Bart's zombie story, she says "awww" but her mouth doesn't open. (00:20:25)
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: When Homer is in Lisa's classroom and imitates a farting sound, there is a sink at the back of the classroom and nobody is using it. However in the wide shot, there are suddenly two people by the sink. When the shot is back to Homer, nobody is by it. (00:03:00)
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