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.
Margical History Tour - S15-E11
Plot hole: During the Mozart skit, Lisa puts a sleeping tonic in the emperor's wine, but when she drank it she fell asleep also. How could that be if she only poisoned one glass of wine and it was the emperor's?
Continuity mistake: Throughout the whole of this episode, the picture keeps on changing on Mr. Burns' wall. Sometimes even in the same scene.
Milhouse Doesn't Live Here Anymore - S15-E12
Continuity mistake: When Homer pulls up at the traffic lights, he stops over both of the lines indicating the crossing. When the woman gives him money in the next shot, he is behind the lines.
Continuity mistake: In the Itchy and Scratchy cartoon 'A Briss Before Dying', Itchy throws Scratchy's eyes out of the window, but after putting the rest of his body through the grinder the eyes are back.
Continuity mistake: When Krusty is sacked, his bottle of pop vanishes. (00:13:45)
My Mother the Carjacker - S15-E2
Continuity mistake: When Homer is looking for a message to stop Grandma Simpson and the jail bus on the electric sign he has Bart with him. Then Bart is not on the bus with Homer and Grandma, but he is back at the end of the episode.
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: 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)
I, (Annoyed Grunt)-bot - S15-E9
Continuity mistake: The jagged piece of metal that Homer pulls from his arm disappears from the bedside table in the very next shot. (00:16:30)
Continuity mistake: After Homer pushes in front of Lisa in the queue for the bathroom, she loses the toothbrush she was holding in her left hand. (00:01:15)
Treehouse of Horror XIV - S15-E1
Continuity mistake: The piles of sweets and candy that Bart and Lisa have at the start vanish in the wide shots when Homer arrives.
Continuity mistake: When Krusty enters the Jewish Walk of Fame building, there are no stars on the pavement outside it. When he leaves there are. (00:06:15)
Continuity mistake: When Homer writes an autograph for Ralph at the restaurant, Lisa is sitting opposite Marge, has yellow liquid in her glass and has red and brown food on her plate. In the next shot, Lisa is opposite Homer, has a blue drink and has white and green food. (00:14:25)
Continuity mistake: When Homer leaves the studio after being cancelled there is no sign next to the door. When Marge walks up to comfort him there is now a 'stage door' sign there. (00:18:35)
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)
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.
Diatribe of a Mad Housewife - S15-E10
Continuity mistake: During the "performance" at the end of Marge's book, "Ned" is skewered by a harpoon and falls off the cliff. When "Homer" walks up to the cliff edge you can no longer see the rope trailing from the end of the harpoon. However, moments later he gets his leg caught up in it and is pulled over himself.
Margical History Tour - S15-E11
Factual error: Lisa, as Salieri, says she has wine for sale : "Claret, Port, Riesling ...". All three glasses on her tray are filled with red wine. Riesling is a white wine.
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.
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