Continuity mistake: When Burns hangs up the phone to Kent Brockman, his name tag on his radiation suit disappears. (00:10:20)
Continuity mistake: When the purple jam pops on the core temperature dial, it leaves some residue on the dial which has disappeared in the very next shot. (00:05:10)
Continuity mistake: When Homer is talking to Lenny and Carl about the movie his pink doughnut disappears from his hand. (00:03:45)
Continuity mistake: The colour of Bart's plate at the BBQ changes between shots. It goes from dark blue to light blue. (00:07:40)
Continuity mistake: When Homer and Ned are talking after Homer tries to return the BBQ, the BBQ moves place on the lawn. Note the stepping stones. (00:17:40)
Continuity mistake: At the end Todd's shirt changes from white to red. (00:21:15)
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)
Burns Verkaufen der Kraftwerk - S3-E11
Revealing mistake: When Homer takes out the dollar bill from his back pocket right before he puts it into the vending machine, the left side of the screen is cut off a little. You can tell especially with how the trash bin in the bottom left has a splotchy white line next to it. (00:02:20)
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