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)
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)
Continuity mistake: When Lisa is explaining to Marge why she is unhappy, the door to her bedroom closes on its own. (00:07:10)
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.
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)
Continuity mistake: When Homer and Bart are fighting in the bathroom, in one shot we see the shower knock some bottles from the shelf on the wall, but then straight in the next shot they're back on the wall. (00:00:50)
Continuity mistake: Lisa's Malibu Stacy dollhouse disappears from the rug after Maggie spells 'price' with her blocks. (00:03:55)
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