Bart's Friend Falls in Love - S3-E23
Continuity mistake: When Bart admits to Milhouse that he narked on him and Samantha he is facing the bed. When the camera changes, he has turned 90° and is no longer looking at the bed. (00:19:00)
Bart's Friend Falls in Love - S3-E23
Continuity mistake: When Millhouse asks Samantha if he can walk her home, his glasses are well above his ear. As the camera changes his glasses are now lower. (00:07:20)
Continuity mistake: Bart's guitar disappears when Otto starts to race to school. (00:11:15)
Continuity mistake: When the band is talking to the audience about being the 6th member of Spinal Tap, the water on the stage simply vanishes between shots. (00:06:55)
Continuity mistake: When we see Homer in the car for the 1st time since the band hit the stage, he has his hand up in the exterior shot but in his bag of crisps from the interior shot. (00:05:20)
Other mistake: At the concert when Bart throws the Frisbee into the dude sat next to him, the crowd in the background are all missing their eyes. (00:03:50)
Continuity mistake: When Marge tells Otto to get a job, the books on the bookcase by Otto change/move between shots. (00:16:50)
Continuity mistake: When Otto crashes the bus, there is no sticker on the bumper. When the bumper flies off, a sticker has appeared. (00:11:30)
Continuity mistake: Selma says she wants a separation while leaning forward with her arms behind her. Camera changes and she is stood with her arms closed. (00:18:40)
Continuity mistake: When Bart realises Selma has 1 hour to live, the remote Patty is holding disappears as the camera zooms into Bart. (00:17:50)
Continuity mistake: When Marge spots Krusty, from the regular view, Krusty's hand is down on the table. But from the view from the camcorder, his hand is on top of the cake. (00:14:00)
Continuity mistake: When Bob is litter picking, the amount of litter by his feet changes drastically between shots. (00:03:20)
Continuity mistake: When Selma tells Bob not to worry about the cost of the wedding she has a cigarette in her hand which disappears almost instantly when she grabs Bob's hand. (00:10:40)
Continuity mistake: The way the chapstick is held out for Bob in prison changes from one shot to the next. (00:04:40)
Continuity mistake: When Bob is telling his story about picking up rubbish on the side of the road, we see Bob in The Front of a line of people. When we see the convicts in the reflection of the trooper's sunglasses there is someone in front of him. (00:03:20)
Continuity mistake: During the meal at the start, the food on Bart's plate changes between shots. (00:02:50)
Continuity mistake: At the dinner table, we see an overhead shot of the table and there is a glass of milk next to Homers hand. As the shot changes, this glass is now gone from that location and moved to nearer Bart. (00:02:50)
Continuity mistake: When Lurleen is in the studio with the Simpson family, Homer has his arms crossed. When Bart asks to do a solo his hands are now in his pockets. (00:14:50)
Continuity mistake: When Marge and Homer are arguing over his boyhood dream, the time on clock jumps forward between shots. (00:13:20)
Continuity mistake: As people throw stuff at the redneck bar, the guy who introduces Lurleen is leaning to his left then instantly to his right between instant cuts. (00:06:10)
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