Continuity mistake: When Selma is on video at the end of the episode, the iguanas's eyes change from yellow to white during the scene, but not between shots.
Continuity mistake: After Krusty is done doing the special promotion commercial, he is given a telegram informing him of the Soviet boycott. However, this is a flashback, and at this point in time Krusty is illiterate (as will later be established in season 1) so he shouldn't be able to read the telegram. (00:12:30)
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.
Continuity mistake: Lionel Hutz runs out of Burns' office leaving both doors open. When Marge says about going, they are closed. (00:20:20)
Continuity mistake: When Homer is talking about Marge getting a job at the dinner table he has a spoon in his hand. Once Marge has finished scowling he doesn't, and there isn't one on the plate either. (00:08:05)
Continuity mistake: When Homer leans back the lamp next to him changes from a red freestanding one to a blue table-top one. (00:02:40)
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: When Homer is about to punish Bart, his ticket suddenly appears in a shirt pocket. (00:15:10)
Continuity mistake: The position of the video in the basket changes between the first two consecutive shots. (00:04:50)
Continuity mistake: The position of the dots on Homer's pool change position. In the close up they're not near the water line but in the long shot they are. (00:03:10)
Continuity mistake: We see the items fall off the roof but a Christmas ornament is not among them. However one lands on the ground with everything else. (00:02:05)
Continuity mistake: When stuff is falling off the roof, in the long shot there is nothing on the roof but in the close up there is. (00:02:05)
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: What the goats are eating changes depending on the shots. (00:10:10)
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: Homer asks Bart if he took the bins out. When Homer walks into the kitchen, the time on clock changes. (00:10:00)
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)
Kamp Krusty - S4-E1
Continuity mistake: When Lisa gets her report card, the big blue "report card" disappears from the card after she puts it down. (00:04:10)
Brother From the Same Planet - S4-E14
Continuity mistake: After Pepi and Tom walk off into the sunset together it cuts to Bart handing Homer another beer, and the picture behind the couch has brown sails over the boat, but next shot it's colored properly with white sails. (00:21:30)
Homer's Triple Bypass - S4-E11
Continuity mistake: When we first see Krusty start talking to Homer, from the rear shot you can see Krusty is missing his pale colour on his face. However, in every other shot, his pale complexion is there. (00:15:10)
Homer's Triple Bypass - S4-E11
Continuity mistake: When we see Lisa in Sunday school, the colour chair she is sitting on changes from blue to brown as she talks about her dad being very sick. (00:14:40)
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