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)
Continuity mistake: When we see Smither daydreaming about Mr Burns coming in through his window at night, the painting on the wall disappears between shots. (00:14:50)
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)
Audio problem: Ms. Krabappel is telling Bart about the laundry list of sicknesses he had to get out of the test. You can see her mouth doesn't match up when she mentions "...and that unfortunate case of rabies." The studio actually changed what Ms. Krabappel originally said which was "and that unfortunate case of "Tourette's Syndrome." [Some reports say that the original line of Tourette's Syndrome was left in syndications in certain areas. On the DVD version the rabies line has been used]. (00:16:00)
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 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: When the wolf runs out of Krusty's studio, the woman who described it changes from having two gloves on to one glove on. (00:16:40)
Continuity mistake: Willie rips off his entire shirt before he fights the wolf, but in the next scene, when he is wrestling the wolf, he is wearing a torn-up shirt. (00:18:40)
Continuity mistake: After Bart is attacked by the wolf and goes back into class, the banner above the chalkboard that has the alphabet on it is blue. Every time we see the banner after that it is yellow. (00:18:40)
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 Bart comes back into the classroom after the wolf attack, Ms Krabappel is writing on the black board and there is quite a bit of writing up there. After we see Bart the shot goes back to Ms Krabappel and all the writing has gone. (00:18:45)
Continuity mistake: When Mr Burns draws the moustache and horns on the T.V. screen of Marge, they just disappear in the next shot when he starts to draw the bees. (00:19:30)
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 leans back the lamp next to him changes from a red freestanding one to a blue table-top one. (00:02:40)
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: You may have to look carefully for this mistake. When Marge picks up the 'Das Butt' video, you can see the cover. In the next shot, you can see Marge holding it so you can't see the cover. But when she hands it over to Ruth, she twists the video round and you can see that nothing is visible. You may have to see it a couple of times. (00:04:50)
Continuity mistake: The position of the video in the basket changes between the first two consecutive shots. (00:04:50)
Audio problem: When Laura insults Kerney and Dolph, Dolph speaks but it's Jimbo's voice we hear even though Jimbo isn't there. (00:05:50)
Continuity mistake: When Marge and Homer are arguing about Don Quixote, Marge's pillow disappears from under her head. (00:09:50)
Continuity mistake: When Homer is sitting in the Frying Dutchman with the crowd gathered round looking at him, the Captain invites people in. Some start walking in, but in the next shot where they are all gathered round again, it is the same group of people again. (00:17:00)
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