Continuity mistake: When Bart falls in the entranceway, a puddle appears under his head between shots. (00:05:30)
Realty Bites - S9-E9
Continuity mistake: The Front door of the Murder House has two windows which vanish, and also a door knob that switches sides so the door opens the opposite way.
Realty Bites - S9-E9
Continuity mistake: Snake has an earring in his right ear, but in some shots the earring is missing then reappears, or it switches to his left ear. Also, when Homer and Snake are fighting on the hood of the car Homer's right shoe falls off, but when the car speeds toward the Murder House, Homer is wearing both of his shoes while he and Snake are still fighting on the hood.
Skinner's Sense of Snow - S12-E8
Continuity mistake: When Skinner dismisses the kids from school, we learn the snow level is near the top of the windows and to the top of the doors. But in the next scene, we see a shot of Springfield and the Simpson home, where the snow level is to the bottom of the door. (00:08:02)
Continuity mistake: Bart is thrown onto his front yard from Sideshow Bob's limo. The campaign buttons pinned to Bart's shirt by Bob's goons in the previous shot have disappeared.
Continuity mistake: Homer draws a face on a punching bag. He starts to show Bart how its done. The punching bag has its eyes filled in now.
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: Apu grabs the ham and crosses out of the expiration date with a thick black marker. When he places the ham on the shelf the black marker cross out is gone and the writing on the pack looks perfectly printed as before.
Continuity mistake: The name card display stand on Homer's desk disappears after Mr. Burns interrupts him.
Continuity mistake: The bikes disappear after Bart and Milhouse arrive at the cave.
Continuity mistake: When Mr Burns and Smithers are standing outside Springfield Stadium, Mr Burns says to Smithers, "Put a little smile on his card" and Smithers is writing it down with a pencil in his left hand. When the Simpsons arrive in the next shot, the pencil has changed into another piece of paper.
Continuity mistake: When Bart says to Homer that he's on "Jumbo-Vision", the man behind Homer is drinking from a paper cup with a straw, but when we see Homer on the screen, the paper cup has disappeared.
Continuity mistake: After Lisa unlocks the bathroom door with the coat hanger, the puddle of "baby blood" (Pepto-Bismol) at her feet disappears in the next shot.
The Trouble With Trillions - S9-E20
Continuity mistake: In the post office when Chief Wiggum is using the megaphone, Otto is seen in the right line as seen from the door but when Kent Brockman attempts to interview him he is magically in the other line, in front of Krusty.
Kamp Krusty - S4-E1
Continuity mistake: Bart holds up his maths book but in the next shot the book is on the desk. (00:05:00)
Continuity mistake: When Moe is wiping the beer glass at the beginning of the episode, the glass is initially smooth but changes to dimpled in the very next shot.
Continuity mistake: When the students have to practice square dancing in the gym, Lisa spots a male student and there is a red circle next to her on the floor where she is standing. When the teacher tells Lisa she has to dance with groundskeeper Willie, the circle has disappeared.
Continuity mistake: When Lisa arrives home after her parents gave the boy a party, she parks her bicycle near the path. When Lisa sees the boy with make-up on his face in the next shot, the bicycle is on the grass.
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