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.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", Homer looks into the mirror and you can see the carpet at the bottom of the steps is coloured pink. But when Homer falls down the steps in the next shot, the carpet is now green.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", Marge goes to see the typewriter and you can see that viewed from behind the paper in the typewriter is crooked, but when the camera angle changes in the next shot, the paper is now straight.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", Smithers is helping Mr Burns to put crates of beer into the car and you can see there is nothing on the lid on the car boot, but in the next shot a wheel appears from nowhere.
Continuity mistake: The cake that Marge bakes for Homer is initially white, but pink icing instantly appears on top between shots.
Continuity mistake: When Ralph comes to see Lisa, she is wearing her turquoise slippers. However, after he leaves and she drops the book on the floor, her slippers have gone.
Factual error: When Selma throws Moleman out of her car, the passenger door is hinged at the rear instead of The Front as it should be on a relatively-modern hatchback, and the window crank is visible on the outside of the door.
Audio problem: After everyone congratulates Ned, he goes to Homer and his mouth moves but nothing is said, and the first line afterwards is not in sync with his mouth movements.
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.
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