Tree House of Horror V - S6-E6
Continuity mistake: In 'Nightmare Cafeteria,' in the opening shot we see Bart standing on a desk in the back row. It cuts to a closeup, and he is now on a desk in the second row from the back, as well as people appearing around him, such as Milhouse, who wasn't even in the classroom in the opening shot. (00:15:25)
Tree House of Horror V - S6-E6
Continuity mistake: In 'Nightmare Cafeteria,' when Lunchlady Doris spills food on Jimbo, it only goes onto him. In the next shot it is also all over the table. (00:16:15)
Tree House of Horror V - S6-E6
Continuity mistake: In 'Nightmare Cafeteria,' Lisa and Bart run into the basement, to tell Marge they are cooking kids at school. Between shots Bart and Lisa swap places. (00:18:15)
Tree House of Horror V - S6-E6
Other mistake: At the end, just before the Simpsons are turned inside-out, Bart is not only wearing his red shirt in bed, but sitting in a position that's not even possible in any position he was sleeping in. (00:20:35)
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", when Homer returns to the basement after his first time travel, there is a scorch mark on the work bench where the toaster was placed when he turned it on. The angle changes to a shot from the stairs, and the scorch mark is gone. Later in the episode, the mark is back on the bench.
Tree House of Horror V - S6-E6
Other mistake: At the speed that Homer was chasing the family outside the lodge at the end of "The Shinning", he would have caught them up with ease.
Tree House of Horror V - S6-E6
Continuity mistake: When Skinner is cooking the kids, he and the cook are chasing Bart, Lisa, and Milhouse. From the back, he has an apron tied around his neck. In the next shot, he puts on an apron.
Tree House of Horror V - S6-E6
Continuity mistake: When the Simpsons shed their skin at the end, the skin falls to the floor. In the next shot it's gone.
Tree House of Horror V - S6-E6
Continuity mistake: In the third story, the five remaining students are all in one classroom - Bart, Lisa, Milhouse, Wendell, and Ralph. Ralph disappears when the other four try to escape.
Tree House of Horror V - S6-E6
Continuity mistake: At the end of 'The Shining, ' the family sits in the snow, Homer holding Willie's portable TV. Later, when they're frozen, Homer is holding nothing, although the TV's snow-topped antenna is still there. Additionally, Homer's left shoulder is missing. This is all due to the lack of correct colorization.
Tree House of Horror V - S6-E6
Continuity mistake: In "Nightmare Cafeteria", Mrs Krabappel sends Wendell into detention and you can see there is a book on her desk. But from an overhead view, the book has gone and there is now paper in front of Mrs Krabappel.
Tree House of Horror V - S6-E6
Continuity mistake: In "Nightmare Cafeteria", when Jimbo Jones is speaking to Bart at the table in the lunchroom, they are initially sitting in front of a cabinet, but in the next shot they are sitting in front of a picture.
Tree House of Horror V - S6-E6
Continuity mistake: In "Nightmare Cafeteria", Lunchlady Doris is speaking to Principal Skinner in the lunchroom and you can see she has nothing in her mouth, but a couple of shots later a cigarette suddenly appears.
Tree House of Horror V - S6-E6
Continuity mistake: In "Nightmare Cafeteria", at the Oktoberfest, Bart and Milhouse are sitting at a table and you can see there is nothing in front of them, but when a waitress comes to their table in the next shot, a chocolate cake suddenly appears.
Tree House of Horror V - S6-E6
Continuity mistake: In "Nightmare Cafeteria", a teacher walks into the classroom and sees Bart sitting at his desk facing backwards. At this point, you can see the lockers behind the teacher are white, but in the next shot they are blue.
Tree House of Horror V - S6-E6
Continuity mistake: In "Nightmare Cafeteria", when Bart is being served his lunch at the lunchroom, his tray is initially plain but in the next shot it is now divided into six squared segments.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", Homer is eating a bag of crisps while sitting on the sofa and you can see "Potato Crisps" logo on the bag is coloured red, but in the next shot it is now blue.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", the family stand up to see Flanders on the television set in front of them at the breakfast table and you can see there is nothing behind Marge on the table, but when the camera angle changes in the next shot, a glass has suddenly appeared.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", when Homer returns after sitting on a fish, he enters the kitchen and you can see the table is completely clear. But when Bart and Lisa (as giants) attempt to kill their father, we cut to Homer and there are plates with food and glasses on the table.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", when the machine picks up the Simpsons' house to take it to the Re-Neducation Centre, the downstairs windows change between shots.
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