Tree House of Horror VI - S7-E6
Continuity mistake: In "Nightmare on Evergreen Terrace", when Marge takes off her oven gloves you can see there is a plate beside her, but when she puts the gloves down in the next shot the plate has disappeared.
Continuity mistake: Bart throws in a microscope in the cupboard of his bedroom when his mother leaves, but in the next shot the microscope is nowhere to be seen.
Continuity mistake: When Marge leaves Bart's room, you can see there is a cap on the mirror on the drawers, but when we see a full view of the room a couple of shots later, the cap has gone.
Continuity mistake: When Bart discovers a pink box by his bed, you can see there is nothing hanging out from the drawer behind him, but in a close-up of Bart in the next shot, a sock suddenly appears.
Continuity mistake: When Homer falls down the stairs, the toy clown lands behind him and a close-up of Homer shows an orange plant pot by the stairs. But when we see a overhead view of Homer lying down, the plant pot is nowhere to be seen.
Continuity mistake: When Marge enters Bart's bedroom, on the wall you can see a picture of a space rocket and on it there is a red cloth. But when we see a close-up of Marge in the next shot, the cloth has gone.
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 "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.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", the family are walking along the corridor at the house with Mr Burns and Smithers and you can see that Homer is walking near a window in one shot, but in the next shot he is now by the railing.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", on the desk next to the typewriter there is a box of paper and some sheets next to it. But when Marge is on the staircase with a baseball bat, the box and sheets have gone.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", when Homer returns after travelling back in time with the dinosaurs, you can see there is nothing on the desk in front of him, but in the next shot a screwdriver appears from nowhere.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", Homer kills a mosquito whilst back in time with the dinosaurs and you can see in the background there is a volcano, but in the next shot the volcano has gone.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", the family are eating breakfast at the table and you can see there is a glass next to Lisa's plate. But when Lisa says "Dad, your hand is jammed in the toaster!" we cut back to Homer and Lisa and the glass has disappeared.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", Homer has his hand jammed into the toaster and you can see Bart has a plate in front of him at the table. But when we see a view of Bart in the next shot, the plate has gone.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", when Homer is sitting at the breakfast table, he asks Marge to pass a doughnut to him and you can see there is a newspaper next to his plate. But when Homer runs off back to the basement, the newspaper has gone.
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