Continuity mistake: In the scene where Bart is in the Court House, when we first see everyone, on the Judges table there is no gavel, yet when the shot changes, a gavel appears. (00:05:20)
My Big Fat Geek Wedding - S15-E17
Continuity mistake: The first shot in the transition from the bachelorette party to the bachelor party shows Skinner drinking beer. He finishes the bottle, and in the next shot it's full again.
My Big Fat Geek Wedding - S15-E17
Continuity mistake: When Milhouse is hit by the rock in class, the map behind him changes colors. This actually happens within the same shot between frames.
Catch 'em If You Can - S15-E18
Continuity mistake: When Homer and Marge think they have finally got away from Bart and Lisa and Homer is about to look in the dinner bowl, on the table holding the food there is a bottle of champagne and two glasses, yet when Bart and Lisa jump out the 3 things have vanished. (00:17:20)
Catch 'em If You Can - S15-E18
Continuity mistake: When Bart, Lisa, Homer and Marge are on the ferris wheel near the end, we can see in one shot the person behind Lisa has blonde hair, and then in the next shot the person has black. (00:18:45)
Catch 'em If You Can - S15-E18
Continuity mistake: The Credit card numbers for Ned and Rod's VIZA cards are exactly the same: 8525 4941 2525 4158 First Bank of Springfield Viza valid from 1/1 to 12/31(thirty years on the credit card?)
Catch 'em If You Can - S15-E18
Factual error: Bart gets bored of "Love Story" and presses the split-screen button to watch Krusty, which replaces the movie audio with the Krusty audio. Split-screen buttons only provide a picture, not an audio.
Catch 'em If You Can - S15-E18
Continuity mistake: When Lisa says "It's called Wicca, and it's empowering" to Bart, her necklace is red instead of white.
Continuity mistake: When Homer goes to work, and Mr. Burns starts complaining about him, we can see that the two pictures are on the far side of the yellow machine on his desk. But then when the shot changes, they are on the other side of the yellow machine. (00:13:45)
Continuity mistake: In the shower scene, we see that in front of Carl, there is a man with brown hair and a moustache in front of him, but then when the shot changes, the man has moved cubicles and a man with white hair has taken his place, and he wasn't even in the previous shot. (00:14:50)
Plot hole: It is shown that Marge first met Luann van Houton when she was ten. But in "Homer Defined", Marge said that she first met Luann in an emergency room when Bart and Milhouse drank paint.
Plot hole: A little ways into the episode, when Milhouse kisses Homer, he says that was the first kiss in his whole life. However, we see him kissing Samantha Stanky in the episode "Bart's Friend Falls in Love," in Season Three.
Continuity mistake: When Marge is saying she would never have married Homer if she had known it was him in the camp, she is on the couch. But in the next shot, Marge is standing up and the cat and dog are on the couch.
Continuity mistake: In the scene where the Simpsons are sitting down to eat dinner, we see Bart pour tomato ketchup all over his potatoes. Then when the shot changes, we can see all the ketchup bottle and the corn has disappeared, and the ketchup has come off his plate. (00:04:50)
Continuity mistake: When Lisa is first delivering her newspaper, Red Dress Press, she unties a pile and the strings are untied. Then the shot changes and the pile has been done up instantly. (00:05:55)
Continuity mistake: When Lisa is interviewing people for her newspaper articles, the order is Nelson, Ralph and then the two purple twins, Sherry and Terrie. But after Ralph has been seen, it is not the twins but a totally different person, Lewis. (00:07:30)
Continuity mistake: Throughout the whole of this episode, the picture keeps on changing on Mr. Burns' wall. Sometimes even in the same scene.
Continuity mistake: Throughout the whole of this episode, the clock on Mr. Burns' desk keeps appearing and disappearing.
Treehouse of Horror XV - S16-E1
Continuity mistake: At the beginning when Kang and Kodos's boss comes to have dinner, his stomach explodes leaving a hole, yet in the next shot the hole has vanished. (00:00:40)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Ned saves Moleman, we see where Ned is standing, there is no manhole beneath him yet when he catches Moleman there is one there. (00:02:30)
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