Continuity mistake: This is a mistake for the introduction from seasons 2-20 (1991-2009). When Homer screams, he turns round. You see this in a wide shot. There are no boxes to the right of the door in the garage. However, two just appear out of thin air when he runs through the garage. (00:01:50)
Continuity mistake: When Bart leaves the school, you can see that his skateboard is green and perfectly flat. But when he is skating through the crowd and past the people at the bus stop, the skateboard has an orange stripe down the middle and is raised at the rear.
Continuity mistake: When Maggie goes through the checkout at the beginning, Marge's trolley is grey and has a red handle. But in the next shot, when Maggie pops out of the bag, the trolley handle is turquoise. (00:00:30)
Continuity mistake: In the introduction, initially Maggie's seat is light purple. However, in the wider shot of her and Marge, the seat changes to a burgundy colour. (00:00:50)
Continuity mistake: In the old main title sequence, when Homer gets out of his car, the door inside the garage is red, but when he is screaming and running towards it, it's now brown. (00:01:40)
Grade School Confidential - S8-E19
Maude Flanders: Excuse me, Edna. I don't think we're talking about love here. We are talking about S-E-X in front of the C-H-I-L-D-R-E-N!
Krusty: Sex Cauldron? I thought they closed that place down?
Other mistake: During the Planet of the Apes musical final number, as the chorus line dances on stage from left to center with the Statue of Liberty in the background; however, parts of the Statue (the pink background between arm and book) cut off the dancer's head as if it is in the foreground. (00:12:19)
Continuity mistake: During the Planet of the Apes musical segment, just before Troy McClure sings "Can I play the piano anymore?", he and Dr. Zaius are standing far apart. In the next shot, they are close together.
Continuity mistake: In the scene at the United Nations, the Russian placard is just a placard. In the next shot, the placard is attached with a rod to rotate it at the push of a button.
Simpsoncalifragilisticexpiala(Annoyed Grunt)cious - S8-E13
Deliberate mistake: During the final musical number, just after Marge sings "I'm going bald from stress," the family sings "But we're happy just the way we are," but Homer's mouth doesn't move, because the producers forgot to record Dan Castellaneta's lines.
Continuity mistake: In the title sequence Mr. Largo has a baton in his hand when conducting. This disappears as he stops conducting to throw Lisa out. (00:00:50)
Character mistake: When Martin Prince supposedly represents Finland's culture for their recreation of the U.N at the Model U.N. Club Martin's outfit is not in fact a Finnish national costume. The outfit he is wearing actually resembles a traditional Sámi outfit. The Sámi people are the indigenous people living in Northern Europe. The specific style of Martin's outfit has characteristics that are more typical to the Sámi on the Swedish side of Sápmi. (00:02:15)
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