Continuity mistake: The antenna from Homer's car disappears when he drops Mother Simpson at the gas station.
Continuity mistake: When Homer is showing his BBQ invites to the kids Bart comes and stands next to Lisa and points to the extra 'B'. In the next shot, he's back over near Homer.
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.
Team Homer - S7-E12
Continuity mistake: When Moe says Mr. Burns is going to have an accident, he's wearing his pin pals shirt. When Moe hits Mr. Burns, he wears a mask and different clothes. Moments later, when talking to his bowling team again, he's back in his pin pals shirt.
Continuity mistake: During the Planet of the Apes musical segment, just before Troy McClure sings "Can I play the piano anymore?", he and Dr. Zaius are standing far apart. In the next shot, they are close together.
Continuity mistake: After Brodka lets Bart go with a warning the boy rides his bicycle home, and in the closeup the handlebar's single brake lever is at Bart's right hand, but in the next wide shot the brake lever is at his left hand. In the closeup there's also a bar at the center of the handlebar, but it's gone in the wide shot. (00:09:05)
Continuity mistake: When Brodka brings Bart to the security office and plays the shoplifters video tape, Troy McClure's trench coat buttons are on his right side (viewer's left), but when Brodka turns off the video the buttons are on Troy's left side, between shots. (00:07:30)
Continuity mistake: At bedtime, while we hear Marge singing to Lisa in her bedroom Bart is lying in his bed, and on Bart's nightstand beside the door there's a framed photo of Krusty and an alarm clock, but in the next closeup the photo has vanished and instead there's a Krusty telephone. (00:15:55)
Continuity mistake: When Marge runs after Bart because he's hiding something under his jacket, Homer pulls out a gate to block Bart's entrance to the TV room. Three different gate styles are shown. (00:20:35)
22 Short Films About Springfield - S7-E21
Continuity mistake: Lisa comes out of the hairdresser's with a new hairstyle which Nelson laughs at. She is walking away to Nelson's right yet when Nelson's being chased by a tall man later and flees around a corner to his left she's suddenly standing there.
Continuity mistake: In the cafeteria scene, the shots from Lisa's perspective show glass protectors with signs where the food is. However, when the shot switches to Lunch lady Doris' perspective, the protectors are gone.
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)
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.
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