Factual error: When you see Ned Flanders put the earth back inside the hole, he puts it back in much faster than is possible. In one shot, he is only just starting to put it back in, then you see Bart who talks to himself for a few seconds, then back to Flanders, who has finished burying what is later to be revealed as the plant that he (Ned) drowned.
Continuity mistake: In the scene where Lisa runs into the attic as she dives behind the chest she rattles a bird cage, later when Ned faints you can see that the bird cage has disappeared.
Revealing mistake: When Bart is watching Rod and Todd walk over to Ned Flanders, you can see their reflections on Bart's telescope. Their reflections do not match their actual positions.
Other mistake: When Bart is looking at Doctor Hibbert on the rowing machine, all the pictures on the walls behind him don't have faces on them.
Other mistake: When Bart is looking at space through his telescope, Earth can be seen through the telescope despite him being on Earth.
Lisa's Rival - S6-E2
Continuity mistake: The position of Ralph's Star Wars action figures changes the two times they show his diorama.
Lisa's Rival - S6-E2
Continuity mistake: During the diorama show, they cut to a shot of Milhouse, and his glasses are broken and taped together. But when he's shown a second time, they are back to normal.
Lisa's Rival - S6-E2
Continuity mistake: When Nelson does his "Grapes of Wrath" demonstration, it splatters all over Skinner's jacket. When he walks over to the rest of the dioramas his jacket is clean again.
Another Simpson Clip Show - S6-E3
Character mistake: In the first scene, Marge says that it is 9:30pm, but after Homer's flashback, the clock next to Homer reads 11:50.
Continuity mistake: Homer's two hairs over his head are dishevelled when the family arrive at Itchy and Scratchy land due to whatever has happened on the detour - however, it suddenly changes to being like he normally has it in the last frame that shows the family in the car, before the car park is shown.
Continuity mistake: When Cletus takes a picture of one of the robots at the parade, the old lady next to him changes from black to white between shots.
Continuity mistake: Bart is thrown onto his front yard from Sideshow Bob's limo. The campaign buttons pinned to Bart's shirt by Bob's goons in the previous shot have disappeared.
Tree House of Horror V - S6-E6
Audio problem: In 'The Shinning' when the Simpsons finally arrive at the house, they climb out the car and start yelling "Hi." and "We're here." Listen to Lisa's voice, and watch her mouth. The audio is totally out of sync with her mouth movements. (00:02:30)
Tree House of Horror V - S6-E6
Continuity mistake: In 'The Shinning,' Homer is chasing his family with an axe. They are hiding behind a curtain, and then they run outside, away from Homer, opening only one of the double doors. In the following overhead shot of them running through the snow, both doors are now open. (00:07:30)
Tree House of Horror V - S6-E6
Audio problem: In 'Time and Punishment,' Lisa yells "Dad your hand is jammed in the toaster!" and we then hear her screaming in the following shot, however her mouth is closed. (00:08:55)
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)
Tree House of Horror V - S6-E6
Continuity mistake: In 'Time and Punishment,' in the basement, as soon as Homer puts the back of the toaster back on, two pink wires taped to the side of the toaster suddenly appear. (00:09:30)
Tree House of Horror V - S6-E6
Continuity mistake: In 'Time and Punishment,' Homer goes back in time, then returns, and the kitchen table has plates of food, cutlery and glasses all set out. Then when Homer is informed that Ned is the Lord of the Universe, the house starts shaking, but you can see the table is now totally clear. (00:11:15)
Tree House of Horror V - S6-E6
Continuity mistake: In 'Time and Punishment,' when the machine picks up the Simpsons' house, to take it to the 'Re-Neducation Centre,' the garage is on the wrong side of the house. It is on the correct side in the following shots of Homer returning. (00:11:20)
Tree House of Horror V - S6-E6
Continuity mistake: In 'Time and Punishment,' when Homer exits the basement, he discovers Bart and Lisa are giants, and he then screams and runs back to the "basement" - however the same door that leads to the basement seconds ago, now leads to a green -carpeted room, and the basement seems to have vanished. The door is also hinged on the wrong side. (00:12:50)
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