Continuity mistake: When Moe confronts Jimbo, Jimbo has his back in a corner with just a cabinet behind him. When the shot changes he is next to the green chair. (00:19:00)
Continuity mistake: When Bart is out in the hallway being attacked by the wolf, he is missing his right shoe. When he goes into class he is missing his left shoe. (00:18:40)
Continuity mistake: When we see Krusty with a bird on his head, the claws' position on Krusty's head change. Note his left eye. (00:16:20)
Continuity mistake: Smithers opens the second case for Tom Jones as the green gas has all but dissipated from the case. When the shot changes, there is still gas coming from the case. The remaining gas also changes place in the vicinity of the case. (00:15:50)
Continuity mistake: When Tom Jones walks off-stage a towel appears around his neck instantly between instant camera cuts. (00:16:00)
Continuity mistake: When Burns is lay on the floor telling Smithers that he dreamed of Marge again last night, the painting behind Smithers is very badly painted and very childish in appearance. Burns' head is completely disproportionate for example. When the shot changes to Smithers the painting is now a lot more detailed and no longer looks childish. (00:14:42)
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: When Bart places his money on the kiosk, the end of the note is overhanging the edge of the kiosk. When he grabs his money after being refused entry, the entire note is on the kiosk. (00:19:20)
Itchy and Scratchy: The Movie - S4-E6
Continuity mistake: When Bart tries to buy a ticket to Itchy and Scratchy, the booth is unoccupied. When the shot changes the man has appeared in the booth. (00:19:20)
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)
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 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: When Millhouse first spots the spookhouse, there is a red item on the floor by him, as he walks towards the spookhouse the item has gone. (00:03:00)
Continuity mistake: Homer says he wasn't sure if he was doing the right thing in regards to giving up doing to church. When he does he puts his fork down across his knife. When the camera cuts the two utensils are no longer crossed. (00:10:40)
A Streetcar Named Marge - S4-E2
Continuity mistake: When Maggie is looking down at the desk to see the keys, there is a book on the desk as well. When she's looking down at the adult walking into the room the book has gone. (00:14:00)
A Streetcar Named Marge - S4-E2
Continuity mistake: Just as Marge heads out for her 1st audition, the kids go from lying on the sofa to sat up instantly. (00:03:35)
A Streetcar Named Marge - S4-E2
Continuity mistake: When Homer picks up the snacks from the vending machine, 2 items are left on the floor. When Marge is practising her lines a few shots later, the items have disappeared. (00:11:10)
A Streetcar Named Marge - S4-E2
Continuity mistake: When Marge walks across the living room she has a bag in her hand which disappears when the camera changes. (00:03:35)
Marge vs. the Monorail - S4-E12
Continuity mistake: When the sticker reveals the 1964 world fair sticker on the monorail, the world fair sticker is blue. Shot changes to a high angle shot and now the sticker is white. (00:16:40)
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