Tree House of Horror VI - S7-E6
Continuity mistake: In the 'Nightmare on Evergreen Terrace' segment, in Bart's dream you see a shadow of what looks like someone with four knife-fingers. Then when we see who it is, it is Willie holding a rake. But the rake has five tines, not four, so the shadow is wrong. (00:07:00)
Tree House of Horror VI - S7-E6
Continuity mistake: In the 'Nightmare on Evergreen Terrace' segment, in Martin's dream, Willie emerges from the blackboard with a rake which has a wooden handle. When the shot changes the rake handle has changed to being metal. (00:08:25)
Tree House of Horror VI - S7-E6
Continuity mistake: When Professor Frink is explaining where Homer is, in "Homer3", Ned Flanders is leaning behind the couch between Bart and Marge, and Chief Wiggum is standing up. When Homer complains, Ned is between Marge and Chief Wiggum, who is now sitting down on the couch.
Tree House of Horror VI - S7-E6
Continuity mistake: For most of "Homer3", Marge's shoes are red. When Frink explains that Homer is 3rd Dimension, Chief Wiggum says, "Slow down, egghead". During this shot, Marge's shoes are green. When Homer complains, Marge's shoes are back to red. (00:18:05)
Tree House of Horror VI - S7-E6
Factual error: In "Nightmare On Evergreen Terrace", Martin has a dream, in which he speaks Latin. At one point, he says a verb that doesn't exist: "morire". The words is "mori" (to die), "moritur" (noun dies) or "moreris" (you die). The "moirie" is also on the blackboard behind Martin. (00:08:20)
Tree House of Horror VI - S7-E6
Factual error: During the "Attack of the 50 ft. Eye Sores" segment, the family steps into the footprint of the giant donut guy to look at the label. The label should appear to be spelled backwards, but it was not.
Tree House of Horror VI - S7-E6
Continuity mistake: In 'A Nightmare on Evergreen Terrace', when the children are all discussing their dreams, Ralph's sling is blue. After the shot of Principal Skinner, Ralph's sling is white.
Tree House of Horror VI - S7-E6
Continuity mistake: When the advertisement of Kent Brockman comes to life he is a thin cut out from a bill board, however the next time you see him he is in the shape of Kent Brockman and no longer thin.
Tree House of Horror VI - S7-E6
Continuity mistake: During the 'Nightmare on Evergreen Terrace' segment, when Bart is running from Willie, he plans to sink Willie in the sandbox. In the first view of the sandbox it is isolated on a field, yet when Willie is sinking in the sand there is a jungle-gym beside the sandbox. (00:11:35)
Tree House of Horror VI - S7-E6
Continuity mistake: In "Homer3", when we hear Homer saying "I don't have much time" you can see Bart has a policeman next to him, but when we see a close-up of Bart in the next shot, the policeman has gone.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Attack of the 50 Foot Eyesores", when Homer puts the chain in the doughnut of the Land Lad Donuts logo, the position of the chain changes between shots.
Tree House of Horror VI - S7-E6
Continuity mistake: In "Nightmare on Evergreen Terrace", when Bart is hit on the head by the Frisbee, you can see the Frisbee is further away from Bart, but a couple of shots later it is now by his feet.
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.
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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