Continuity mistake: When Bart is trapped in the coffin, the cables leading to the Cadaver Cam TV disappear then reappear. (00:04:10)
Continuity mistake: In the very beginning of the episode, Bart places Santa's Little Helper's dish into the sink and begins playing with the faucets. Marge comes in, and the bowl magically disappears from the sink, and it's missing for the rest of the scene.
New Kids on the Blecch - S12-E14
Continuity mistake: When Bart and the group are singing at school, when we see L.T.Smash, his arms are down then instantly in the next shot, they're on his sides. (00:08:10)
New Kids on the Blecch - S12-E14
Continuity mistake: When L.T.Smash sees his admiral, we see he his has badges on, yet when the shot changes and zooms in on him, they're gone, then they're back. (00:17:10)
New Kids on the Blecch - S12-E14
Continuity mistake: When L.T. Smash switches the lever to Commission from De-Commission, after that, when the shot changes, it says Ship and Museum. (00:18:10)
New Kids on the Blecch - S12-E14
Continuity mistake: When Homer is pinching Lisa with the "jealousy bug", the sign behind him changes from a gift store to an antique store.
New Kids on the Blecch - S12-E14
Audio problem: When the VJ on the TV says "That was the latest ad from Stridex pads. Medicated", her mouth is terribly out of sync with her voice.
Continuity mistake: When Homer sees the hotdog that Duffman holds up near the end of the episode, the order of the condiments on the hotdog change in subsequent shots.
Continuity mistake: Before Homer sees his vision of Caesar Romero the day sign next to him says 9. After the Duff executives see him talking to nobody they decide to take him away and when they get down there the sign now says 12 with the president of Duff confirming it in his speech. Doubtful it took them 3 days to get down to him and not much reason for Homer to take 3 days off the sign seeing as he was almost passing out from the lack of food.
Continuity mistake: When Homer is eating the sausage, a square on his shirt disappears in a shot from his feet and then is back in the next shot.
Other mistake: When Lisa says Bart's got a girlfriend, Bart says "no, I don't, that's the problem", his neck becomes longer and starts to wobble, a problem with the animation.
Continuity mistake: Lenny's mug refills itself when he tells Homer, "You want to help me?"
Continuity mistake: When Marge is putting Lisa and Bart's clothes on, she puts Lisa's dress on Bart and Bart's shorts and t-shirt on Lisa, but 2 seconds later, when they are entering the kitchen, they are wearing their normal clothes.
Continuity mistake: There are two problems with Lisa's Indian burns. When we first see them, she has 4 burns. Then, while she is showing them to the bullies, there are 3, then 4 again. Then, in the very next scene when she is walking with Nelson in the schoolyard, they are gone altogether.
Continuity mistake: When Lisa is watching tapes of Francine's bullying, there is a scene where they are at the pool. Francine is wearing nose clips in this scene. But when Lisa freeze-frames and leans in to closer inspect the shot, Francine's face is shown on the screen without nose clips. They appear again when Lisa exclaims "Of course. The nose clips."
Continuity mistake: When Marge is racing the school bus, as the bus jumps out of the ditch you can see that the bottom of the bus is painted to resemble a skateboard. When the bus lands from this jump, however, the bottom of the bus is an even grey colour.
Audio problem: In the scene where Lisa is presenting her bully repellant at the 'Big Science Thing', when she first walks onstage there are murmurs of disapproval among the scientists, in which you can hear one of them saying, "Let's not listen". But after Lisa sprays the repellant on herself and stops Francine in her tracks, the scientists start murmuring in astonishment. However, if you listen closely, you'll hear the same recording of someone saying, "Let's not listen".
Continuity mistake: Towards the end, when the scientist is trying to give everybody diamonds, a shot cuts to just Marge and Lisa, but out of nowhere Maggie appears. (00:19:30)
Continuity mistake: When they are riding the boat type raft, the stripes change from red on the left and black on the right, to the other way around and then back again.
Continuity mistake: When Bart is eating the grubs, his tongue appears to be green, in all other shots it's back to red.
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