Continuity mistake: The Simpsons' bathroom has changed position. In 'Today I am Klown' it was opposite the top of the stairs, at the back of the house; in this episode it is at The Front of the house. (00:00:35)
I, (Annoyed Grunt)-bot - S15-E9
Other mistake: When the bullies are riding around Bart and Milhouse there are problems with their water bottles. In some frames they vanish, in some others the bullies' far legs are drawn in front of the bottle. (00:01:40)
Continuity mistake: When Krusty ends up at his old neighbourhood, the street changes between shots. E.g. the "LLBeanie" sign appears above the door of the 2nd shop and the suit of the man reading the newspaper changes from grey to blue. (00:05:25)
Continuity mistake: After he takes a puppy from Bart, Snake's car appears from nowhere. (00:04:15)
Continuity mistake: Right at the very end scene, when Lisa and Maggie are sitting on the bed, you can see that there are 6 bricks on the bed. Yet when the shot changes there is only one behind Maggie, meaning two of them have just vanished. (00:20:30)
The Ziff Who Came to Dinner - S15-E14
Continuity mistake: In once scene, Artie walks past the Simpson's to Selma (or Patty). In the next shot he is shown walking past the Simpson's again.
I, (Annoyed Grunt)-bot - S15-E9
Continuity mistake: At one point in the cycle shop, Homer has his hand in his pocket in one shot, and it's suddenly out in the next one.
Margical History Tour - S15-E11
Continuity mistake: Lisa empties a glass of red wine that she previously poisoned with sleeping potion. When she falls over sound asleep, her glass is shown on the ground next to spilled wine.
Continuity mistake: When Lisa is explaining to Marge why she is unhappy, the door to her bedroom closes on its own. (00:07:10)
The Ziff Who Came to Dinner - S15-E14
Continuity mistake: When the Simpsons first see Artie Ziff in the attic, as he stands up, there's a theremin behind him (corresponding with Homer's comment that the "ghost" was playing his theremin). Artie puts a sign up above it. However, when Artie moves to the rafters, the sign he put up is still there, but the theremin isn't.
Continuity mistake: The coathanger is too small to reach the medicine cabinet.
Continuity mistake: Throughout the whole of this episode, the clock on Mr. Burns' desk keeps appearing and disappearing.
Diatribe of a Mad Housewife - S15-E10
Continuity mistake: After the clown balloon that rips at the car dealership there is a bit of material left over and there is also a cord attached to the wooden platform where the balloon was. After a reaction shot, it cuts back and the material and the cord have both disappeared.
Continuity mistake: In the scene where Homer is going down the escalator, in the Costington's Centre, after he gets on two men follow him, then a woman, yet when the shot changes, it's a man then a woman then the other man. (00:00:45)
Factual error: The tongue Lisa sleeps on in the museum isn't anatomically correct. It shows the bitter and sweet taste buds as being side by side on the surface of the tongue. In reality, different taste buds are not in specific places on the tongue. I wouldn't normally complain, but this is supposed to be in a museum.
Marge vs. Singles, Seniors, Childless Couples and Teens, and Gays - S15-E8
Continuity mistake: When we see The Town at the meeting, Smithers is sitting in front of Selma. In the next shot, though, Smithers is suddenly on stage holding the 'SSCCATAGAPP' sign and Lenny is sitting in front of Selma. (00:08:50)
My Mother the Carjacker - S15-E2
Other mistake: In the scene where the prisoners are singing along with Mona, two inmates have the same Identity number.
Continuity mistake: When Lisa is standing outside the Kwik-E-Mart, the "open 24 hours" sign on the window changes position between shots. (00:13:00)
Treehouse of Horror XIV - S15-E1
Continuity mistake: When the younger Professor Frink is receiving and wearing his Nobel Prize, his father goes nuts and starts stealing people's brains. The younger Frink figures out that kicking the elder Frink in the crotch will stop him. When he does, and the elder Frink falls off the stage, he is wearing the Nobel Prize, not the younger. (00:15:00)
My Mother the Carjacker - S15-E2
Continuity mistake: At the end of the episode when Homer is looking for another message from his mom to find out if she's really alive, Marge and the kids enter the kitchen, Marge says that it's 2 in the morning. But the clock on the wall shows that it's 9 pm.
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