Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", the family are walking along the corridor at the house with Mr Burns and Smithers and you can see that Homer is walking near a window in one shot, but in the next shot he is now by the railing.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", on the desk next to the typewriter there is a box of paper and some sheets next to it. But when Marge is on the staircase with a baseball bat, the box and sheets have gone.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", when Homer returns after travelling back in time with the dinosaurs, you can see there is nothing on the desk in front of him, but in the next shot a screwdriver appears from nowhere.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", Homer kills a mosquito whilst back in time with the dinosaurs and you can see in the background there is a volcano, but in the next shot the volcano has gone.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", the family are eating breakfast at the table and you can see there is a glass next to Lisa's plate. But when Lisa says "Dad, your hand is jammed in the toaster!" we cut back to Homer and Lisa and the glass has disappeared.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", Homer has his hand jammed into the toaster and you can see Bart has a plate in front of him at the table. But when we see a view of Bart in the next shot, the plate has gone.
Tree House of Horror V - S6-E6
Continuity mistake: In "Time and Punishment", when Homer is sitting at the breakfast table, he asks Marge to pass a doughnut to him and you can see there is a newspaper next to his plate. But when Homer runs off back to the basement, the newspaper has gone.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", Homer looks into the mirror and you can see the carpet at the bottom of the steps is coloured pink. But when Homer falls down the steps in the next shot, the carpet is now green.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", Marge goes to see the typewriter and you can see that viewed from behind the paper in the typewriter is crooked, but when the camera angle changes in the next shot, the paper is now straight.
Tree House of Horror V - S6-E6
Continuity mistake: In "The Shinning", Smithers is helping Mr Burns to put crates of beer into the car and you can see there is nothing on the lid on the car boot, but in the next shot a wheel appears from nowhere.
Continuity mistake: The cake that Marge bakes for Homer is initially white, but pink icing instantly appears on top between shots.
Continuity mistake: When Ralph comes to see Lisa, she is wearing her turquoise slippers. However, after he leaves and she drops the book on the floor, her slippers have gone.
Continuity mistake: When Homer is eating the sausage, a square on his shirt disappears in a shot from his feet and then is back in the next shot.
Bart Gets an Elephant - S5-E17
Continuity mistake: Bart's elephant crushes the dog house, then it is shown standing, and then it is crushed again.
Dude, Where's My Ranch? - S14-E18
Continuity mistake: When Lisa goes to save Clara, she's wearing the same dress she took to the dance, but after Clara walks safely back to land, Lisa is wearing another outfit entirely, then changes back to her previous outfit in the very next shot, then back to her unknown outfit seconds later.
The Man in the Blue Flannel Pants - S23-E7
Continuity mistake: While Krusty is talking to his merchandising executives, after Krusty says "Get to the point, unless you want a face full of clown chunks!" part of his collar turns green.
Continuity mistake: In Apu's van, one of the octuplets on the left is wearing blue pyjamas while the lullaby is playing, then changes to pink pyjamas after Apu takes the disk out.
Postcards from the Wedge - S21-E14
Continuity mistake: When Lisa is talking to Bart in her room, Lisa's necklace is red rather than white when Bart shouts "End all life on this planet!" (00:08:05)
Continuity mistake: When Carl and Lenny are lowering Homer onto the train window, Lenny's five o'clock shadow is missing.
Rednecks and Broomsticks - S21-E7
Continuity mistake: When the Simpsons are on the highway, Apu's car is seen, and some of the octuplets have yellow skin and blonde hair.
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