Continuity mistake: When the newspapers fall on Skinner, the rake in the corner has turned into a shovel. (00:19:10)
Continuity mistake: When Mr Burns draws the moustache and horns on the T.V. screen of Marge, they just disappear in the next shot when he starts to draw the bees. (00:19:30)
Continuity mistake: After Bart is attacked by the wolf and goes back into class, the banner above the chalkboard that has the alphabet on it is blue. Every time we see the banner after that it is yellow. (00:18:40)
Tree House of Horror V - S6-E6
Continuity mistake: In 'Time and Punishment,' the blue button on the toaster occasionally differs from being at the bottom of the lever to suddenly at the top, visible in the first shots of Homer fixing the toaster and between shots after Homer sits on the pink fish. (00:09:25 - 00:12:45)
Continuity mistake: When Homer drives his car to work it is lime green instead of the usual purplish color.
Tree House of Horror XII - S13-E1
Continuity mistake: At the show the school puts on in "Wiz Kids", when the Montymort Dragon is first seen in the cage, the cage has 6 bars at The Front. When Lisa says "Release The Dragon", the cage has 4 bars. When the cage door is being raised, it has 6 bars again. (00:18:45)
Burns Verkaufen der Kraftwerk - S3-E11
Continuity mistake: As Mr. Burns and Smithers are talking on the phone, we see a hand holding the receiver, and when they're finished, the next shot shows us a black skinned referee. His skin color was yellow before. Also, the phone is nowhere to be seen. (00:15:55)
Continuity mistake: As Homer watches TV at the beginning, if you look really closely, you can see the remote on the couch colored to be exactly like brown. It then disappears eventually, and then reappears. (00:01:35 - 00:02:30)
Continuity mistake: In the scene where Homer dodges the cannon ball, he is wearing goggles and a helmet. But when Homer jumps down from the platform and hugs Marge, his helmet and goggles have disappeared.
Like Father, Like Klown - S3-E6
Continuity mistake: During dinner, Krusty does not have a shirt pocket until he pulls out his handkerchief. (00:06:45)
Homer Badman - S6-E9
Revealing mistake: In the scene where Marge's coat pocket bursts open, the candy flows onto the floor. However if you look to the right of her some of the candy seems to almost float above the floor. The candy's direction is completely to the right but because of the floor being angled it spreads over one woman who is standing and there is nothing there to support the candy. (00:04:30)
Homer's Triple Bypass - S4-E11
Continuity mistake: When Homer is eating the bacon and what looks like scrambled eggs after claiming Marge's oatmeal had a bug on it, the close up of the bacon shows that it is pink/purple and the eggs are yellow but the in the following shot, when Bart says, "Dad, there's a bug on that." the bacon and the eggs switch colours. (00:03:40)
Continuity mistake: When Barney enters the tavern, we see a guy working with Moe. His hands change position between shots. (00:16:15)
Continuity mistake: There are many mistakes when Homer is checking for donut flavors, i.e. the positions of the donuts, the topping, etc. (00:03:50)
Continuity mistake: Mr. Burns gives Marge a present when she starts her new job. The present is light purple with a red ribbon around it. Seconds later though, the present turns to orange with a red ribbon around it. (00:11:45)
Continuity mistake: When Bart comes back into the classroom after the wolf attack, Ms Krabappel is writing on the black board and there is quite a bit of writing up there. After we see Bart the shot goes back to Ms Krabappel and all the writing has gone. (00:18:45)
Mr. Lisa Goes to Washington - S3-E2
Factual error: When the piano keys are shown when it is played at the end of the episode, it is shown having one black key each time. Real pianos start off with three black keys, then two, then three and so on.
Homer's Triple Bypass - S4-E11
Continuity mistake: When Homer is having his operation, and Dr. Nick says, "I know I'm supposed to cut something, but what and where?" his moustache disappears three times. (00:19:50)
Revealing mistake: When Skinner starts to drive the bus, he says "Off we go". During this shot the sky is a brighter colour when you look out of the bus windows than if you just look outside the bus. (00:12:15)
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