Home Sweet Home-Dum-Diddly Doodly - S7-E3
Continuity mistake: At the Flanders' house, in the kitchen, Bart and Lisa have sandwiches on plates in front of them. The shot changes and everyone on the table has an empty bowl including Bart and Lisa.
Continuity mistake: After the "sponge incident," Bart is standing in the drive with the sponge packet in his hand. In the next shot he says "everything about me must go," but the packet has vanished from his hand.
Continuity mistake: After Moe yells at Ralph, there's a woman in the background staring at Moe with an angry look on her face. She turns her head away from him after some seconds, but in the very next shot, she is looking at Moe with the same expression on her face.
Continuity mistake: When Milhouse snitches on Bart, Martin has grey hair and a black suit. In the very next shot he has his ordinary hair and is wearing a brown suit.
Continuity mistake: Jasper's beard is missing when everyone is singing "In-A-Gadda-Da-Vida" at church.
Continuity mistake: When Moe tries to convince Dr Hibbert that Moe's is a family restaurant, Carl's skin is yellow.
Continuity mistake: At the BBQ, Homer lights the grill on fire with a match. When Lisa walks out with the bowl of gazpacho, the fire instantly disappears. (00:13:05)
Continuity mistake: When Bart visits the kid's train ride, the first shot of it shows that it starts with a big curve before the tunnel. When he goes on it the track is straight before the tunnel.
Continuity mistake: When we first see Homer feeding the can to the goat the background shows a wooden coop. In the next shot, a purple tree stump appears there too.
Continuity mistake: When Lisa is first seen petting the baby lamb, she is next to the fence with hay beside her. In the next shot, she's far away from the fence and there's no hay anywhere.
Continuity mistake: After Homer prepares the BBQ he holds out the steak and holds it back in as Lisa speaks. When the shot changes he's holding it out again, and the pile of meat behind him is smaller and has moved slightly under the window.
Continuity mistake: During dinner Marge has no glass to her right. When she says 'Why thank you Homie' a glass has appeared next to her.
Continuity mistake: Throughout the dinner scene, whenever Homer wants the syrup, it disappears and re-appears next to Lisa.
Continuity mistake: Lisa places her Gaspacho on the house end of the table but in a following shot it's at the other end when the dog is drinking it.
Continuity mistake: When Skinner is addressing the class before the meat video we first see him with black trays on the desk to the far right. In the next shot they're much closer to him.
Continuity mistake: When Lisa knocks Bart's chop out his mouth she knocks it out front, however, the chop ends up going to the far right and into Homer's mouth.
Continuity mistake: When Homer is talking to Ned about having his own BBQ Ned is facing the fence but in the next shot he is facing alongside it with his head turned to Homer.
Continuity mistake: On Lisa's plate at dinner she has no mashed potato. During the lamb vision, her pork chops are now smaller and there's now mashed potato on her plate.
Continuity mistake: When Lisa knocks the lamb chop out of Bart's mouth to Homer's it changes colour during the journey.
Continuity mistake: In the Meat video Jimmy's steak is first seen to be a little thin. In the closeup it's very thick, then thin again 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