Tree House of Horror XII - S13-E1
Continuity mistake: In "House of Whacks", when Marge opens the door to the robotic salesman, you can see there is a green mat under the door. But when the salesman puts his foot between the door, the mat is missing, and in next shot the mat has re-appeared.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when the leprechaun knocks Marge's glass over on the table when she is eating her pancakes, the glass changes position between shots.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Homer puts his spade in the ground when he is digging a hole with Bart, the spade keeps disappearing and re-appearing between shots.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Lenny and Carl are crushed by the helicopter at the tavern, the liquid behind Lenny is white but changes to brown in the next shot.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Homer, Bart, Lisa and Maggie are eating their breakfast at the table, they see Marge with a blue beard and Homer spits some food on the table. In the next shot, the food has gone, and when Homer goes to strangle Bart, it is back again.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when the Fortune Teller is speaking to Homer in her office, you can see the crystal ball is in front of her but in the next shot it has moved to the edge of the table.
Tree House of Horror XII - S13-E1
Continuity mistake: In "Hex and the City", when Marge, Bart and Lisa are at the Fortune Teller's office, you can see the Fortune Teller has three cards in front of her on the table. When Homer arrives, the cards have gone, and later on in the scene they are back on the table again.
Continuity mistake: When Judge Harm is yelling at Marge and Homer after they have cut the rope, Marge is wearing lip gloss, and between two shots, it disappears.
Continuity mistake: While Bart is watching wrestling as Marge and Homer are struggling to do the dishes, the buzz cola can he is holding suddenly switches hands between two shots as Lisa is talking to him.
Continuity mistake: When Marge and Homer are put in stocks, Marge initially has lipstick on, but in subsequent courtroom scenes it has disappeared.
Continuity mistake: The stocks that are put on Marge and Homer are fitted so that their head and hands are secured and cannot be removed. Whilst in Flanders garage and Homer is using the rotary saw to remove Marge's stock, Homer's stock is still secured on him but his arms and shoulders are through the stocks.
Continuity mistake: When Homer is sleeping on Bart's school floor, Bart puts his shoe in Homer's mouth. Later, when Homer is screaming about "Cobras," the shoe has been coloured black, rather than the usual blue.
Audio problem: When Homer says "He sees a thousand times worse on that animal channel", his mouth is out of sync when he says "channel."
Continuity mistake: When Bart is digging his hole, Homer goes over and starts talking to him, and you can see a spade standing up outside of Bart's hole. The spade wasn't there in the previous shot of Bart digging. (00:02:05)
Continuity mistake: At the thanksgiving dinner, Lisa tears one of the legs off of the tofu turkey, but in a later shot the leg is back on the turkey. (00:20:20)
Continuity mistake: When Homer is having the thanksgiving dinner at Moe's, he isn't wearing a tie, but between shots he suddenly gains a tie. (00:20:35)
Continuity mistake: At the end of the episode, Homer puts twenty dollars into Moe's tips jar, but in the next shot the twenty-dollar note has disappeared. (00:20:40)
Continuity mistake: When Moe picks up the "Malaysian Beer" and says they don't serve Duff anymore and how good it is, you then see a shot of Homer talking, then when Moe has put down the beer, it's a Duff .
Continuity mistake: In Moe's, Homer points at Lenny and says "Are you calling me a liar," with the same hand which is instantly holding a beer 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.
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