Fat Man and Little Boy - S16-E5
Continuity mistake: When Bart is playing with his toys, we see that in one shot on the shelf there are 4 books, then in the next shot there are 5. (00:03:50)
Fat Man and Little Boy - S16-E5
Continuity mistake: During the scene when the toys are talking about insurance, the design on the arm of the blue jacket changes in different shots. (00:04:00)
Fat Man and Little Boy - S16-E5
Continuity mistake: When Lou and Eddie, the police officers, take Bart's shirts, they both have blue shirts on top, then when the shot changes, one of them has an orange shirt on top. (00:07:40)
Fat Man and Little Boy - S16-E5
Continuity mistake: When Homer goes to see Mr. Gladwell, in one shot, there is no plastic vomit, then a few shots later there is. (00:19:45)
Fat Man and Little Boy - S16-E5
Continuity mistake: In "Fat Man and Little Boy," Homer, while playing with Malibu Stacy, picks up Lisa and runs away from The Front door, which is behind him. In the next shot, however, Homer runs out of The Front door and into the street.
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