Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: On the book on conspiracy theories that Bart buys, there's a picture on the back of the author. The picture changes several times throughout the episode.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: When Homer and Grandpa are trying to sell the tonic at the mall, when Homer is trying to sell tonic to the Critical Man, all the tonic bottles on the table are actually empty. In the next shot, they're full.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: When Lisa jumps off the tyre swing, her dress is fairly dirty. When Homer drives off, the bottom is dirtier than before.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: At the old Simpson home, Grandpa and Homer run into each other and collide on the porch. In this shot, the windows are dark, but in the next, all the windows are instantly lit up by the fire, which would not be possible in the given time.
Continuity mistake: When the snake from the cashier bites Moe, the bites on his arm never change and only remain in the one spot even though the snake bit several different spots on his arm.
Continuity mistake: In the scene in the airport bar, two pilots sitting next to each other have brown and gray hair, respectively. When they stand up, the hair has switched colours.
Continuity mistake: On the plane, where Marge is starting to act up due to her Fear of Flying, Homer and Marge's seat-belts have mysteriously disappeared.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: When the janitor shows Marge and Homer into the Utility Room, the only equipment in there is a couple of mops. Later, the janitor comes back and pulls out a big vacuum cleaner that we never saw previously.
Grandpa vs. Sexual Inadequacy - S6-E10
Continuity mistake: At the retirement home, Grandpa pulls out some medicine from his cupboard. In the next shot, another medicine bottle that wasn't in the previous shots appears.
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.
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.
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