Continuity mistake: When we see Maude ringing items up on the cash register, it has no external display. When Ned is ringing items up there is an external display. (00:20:45)
Continuity mistake: When Ned and Homer walk away from Ned's car, they are by the flower beds. Camera cuts and they are back by the car. (00:18:00)
Continuity mistake: When Ned starts talking about the Leftorium during his barbecue, Marge's dress is pink in one shot. (00:04:05)
Continuity mistake: When Bart taunts Homer, he has nothing on his plate apart from his burger. When Homer runs over a second later there is more stuff on his plate. (00:03:30)
Continuity mistake: When Homer is hanging out of the window there is a blue curtain tie visible however this disappears in the long shot. (00:03:30)
Continuity mistake: When Homer is on the sofa, the cushion under his head disappears when he wakes up. (00:03:00)
Continuity mistake: When the paper falls on Skinner there are blue and white tiles in the doorway, when he is hanging from the hoover, the floor is completely blue. (00:19:20)
Continuity mistake: When Bart draws Skinner before he is declared missing, the paper is flat on the desk in the first shot and held upright in the next. (00:14:05)
Continuity mistake: After Itchy and Scratchy we see Bart and the mob sat in the bar, the mobster at the end of the bar has his drink in 1 hand or 2 hands depending on the shot. (00:08:16)
Continuity mistake: Bart clears some dust off the picture of the Manhattan, once he's made the drink a few seconds later all the dust has gone. (00:06:40)
Continuity mistake: The drink above the Manhattan on the board changes name and picture. (00:06:40)
Continuity mistake: When Bart falls in the entranceway, a puddle appears under his head between shots. (00:05:30)
Revealing mistake: When Bart is hanging from the play bars his hand goes through the bars. (00:02:35)
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)
Kamp Krusty - S4-E1
Continuity mistake: When Lisa is reading her report card, the writing suddenly appears on the back of the paper, even though she didn't flip the card. (00:04:10)
Continuity mistake: When everyone discovers the monster is a boar, there is a Chippos packet on its tusk. But when the boar is shown licking the slime, the Chippos packet has disappeared without ever being touched.
Continuity mistake: Just before Homer meets Bill Gates, and during the meeting, the pens and pencils in the butter alternate between shots.
Continuity mistake: Throughout the trial, the twins keep switching places between shots. Note - one twin has a bow in her hair and the other twin doesn't.
Continuity mistake: At the start of the trial, Nelson is seen sitting in front of Martin and one of the twins. But in the shot after Lisa says Milhouse is the only witness, there is no sign of Nelson.
Continuity mistake: When Bart says "No talking in the cage", both him and Milhouse are right next to each other. But two shots after Lisa says Milhouse is the only witness, Milhouse is a good 10 foot away from Bart without ever having moved the cage he was in.
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