Tree House of Horror VI - S7-E6
Continuity mistake: In "Attack of the 50 Foot Eyesores", Homer enters the Lard Lad Donuts and you can see there is a straw holder on the counter in front of him. In the next shot, the straw holder has gone.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Attack of the 50 Foot Eyesores", when Lisa and Paul Anka perform their duet, you can see behind Anka there is a window and the bottom part of it is broken. But later on it is now completely intact and the two top windows are broken.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Attack of the 50 Foot Eyesores", when the cowboy logo on the Duff billboard comes to life, the position of the people next to the van changes between shots.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Attack of the 50 Foot Eyesores", when Homer answers the door and finds the Lard Lad statue in front of him you can see there is a door knocker on the door and a green doormat on the step. But when he answers the door a second time, the door knocker and doormat have gone.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Nightmare on Evergreen Terrace", when Bart says "You missed a spot" to Willie, the hose next to Bart changes position between shots.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Nightmare on Evergreen Terrace", when Bart and Lisa come into the kitchen to tell their parents about Martin's death, you can see next to Lisa's plate there is only a spoon but in the next shot a fork suddenly appears.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Nightmare on Evergreen Terrace", when Martin dies in the classroom you can see that in the desk next to Bart there is a boy student, but in the next shot he has changed to a girl.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Homer3", when Marge is eating the seashells with Patty and Selma, you can see the wall near the staircase is red in long shots, but in close-ups it changes to pink.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Attack of the 50 Foot Eyesores", when the policeman shoots the football player who comes out of the Big and Tall Men's Shop, the shops change between shots.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Attack of the 50 Foot Eyesores", when Lisa is speaking the executive at the Ad agency you can see the back of Lisa's chair is large but later on it decreases in size.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Attack of the 50 Foot Eyesores", the executive goes to the piano and you can see a penholder on his desk. But the camera angle changes to behind the piano in the next shot, the penholder has moved position and there are now more items on the desk.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Nightmare on Evergreen Terrace", when the teacher hands out the papers before the test, you can see that in the desk sitting next to Bart is a student wearing green sunglasses and has blue hair. In the next shot, Martin is suddenly beside Bart.
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: When Homer is stuck in the hole after saving the plant, Marge is seen in the background wearing lipstick. When Mr. Burns asks if there is anything he can do for them, there is a closeup of Marge and her lipstick is gone. When they cut back to Homer, Marge is again wearing lipstick.
Continuity mistake: When Homer says he's going to the cinema, we see his drinking bird working in front of the computer. It makes a key press on the lower right of the keyboard, however on the following close up shot of the bird pressing the Y key it is a standard qwerty keyboard layout.
Continuity mistake: The canister the hippies use to destroy the germs in Burns lab has the words "antibiotics" appear after the clock rings.
Continuity mistake: When Marge talks to cable guy, the trees and hammock disappear.
Continuity mistake: The moss on Walt Whitman's grave when Homer removes it changes.
Continuity mistake: The antenna from Homer's car disappears when he drops Mother Simpson at the gas station.
Sideshow Bob's Last Gleaming - S7-E9
Continuity mistake: When Maggie (sitting on Marge) appears on "Tyrano-Vision," Homer waves his hand in front of her. However, we can see that he isn't leaning over too far, and in the next shot it is clear that both Bart and Lisa are sitting between Homer and Marge, so there is no way he could have reached over to her in the first place.
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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