Tree House of Horror X - S11-E4
Continuity mistake: In "Desperately Xeeking Xena", as Lucy Lawless grabs the lips of the Collector, you can see a big "C" on the back of his cape. After she has thrown him across the room and he gets up, the "C" is gone. It reappears later in the fight.
Tree House of Horror X - S11-E4
Continuity mistake: When The Simpsons go home they find the words 'I know what You Did' on their door, written in blood. Then another shot after, you can see the door with nothing on it.
Tree House of Horror X - S11-E4
Continuity mistake: In the 'I know what you did-diddly-did' story, the Simpsons car keeps changing from dark purple to the normal pink colour.
Tree House of Horror X - S11-E4
Continuity mistake: In the flashback, the car moves a lot further than it did originally after hitting Ned.
Tree House of Horror X - S11-E4
Continuity mistake: When The collector falls over the banister and into the Lucite, Xena is taped into a plastic pouch; but when she gets out, she pulls the bag down easily, without ripping open the bag or the tape.
Tree House of Horror X - S11-E4
Continuity mistake: When Marge hits Ned Flanders, she pulls over, in the middle of the road. But When they go to see who they've hit, and when Homer picks Ned up and 'talks' to him, the car has disappeared from the road.
Tree House of Horror X - S11-E4
Continuity mistake: While the Simpsons are standing in the line, waiting to enter the rocket to Mars, you can see Professor Frink wearing a white shirt. However when he enters the lift to the rocket, he has a green shirt on.
Tree House of Horror X - S11-E4
Continuity mistake: When the flashback of running over Ned happens his jumper is ripped and sleeveless because he has just been a wolf. Earlier though when the Simpsons find him, his jumper is not ripped.
Tree House of Horror X - S11-E4
Continuity mistake: In "I Know What You Diddly-Iddly Did", there are a few errors: Homer says, "Guess I forgot to put the fog lights in" at a slightly higher pitch to when we heard him say it earlier (When the event actually happened). When Marge actually ran over Ned she did not carry on driving. The flashback shows that Ned was run over without glasses. However in the actual event he had glasses on.
Tree House of Horror X - S11-E4
Continuity mistake: In "I Know What You Diddly-Iddly Did", just after Ned has been run over, he turns from a werewolf into his normal self. If you look carefully, his left ear turns brown for a second before turning yellow again.
Tree House of Horror X - S11-E4
Continuity mistake: When the possible witnesses of who may have seen the Simpsons kill Flanders, Homer is shown pointing at Marge with his right hand, but the next shot shows him pointing with his left hand.
Tree House of Horror X - S11-E4
Continuity mistake: In "I Know What You Diddly-Iddly Did", Marge's necklace keeps changing from red to white when the Simpsons are at church, and when they go back home.
Hello Gutter, Hello Fadder - S11-E6
Continuity mistake: When Homer is talking to Ron Howard on the street, there is no pocket on Homer's shirt. But when he takes the money from Ron, he puts the money in a pocket that suddenly appears.
Continuity mistake: In the scene after Homer and Apu rescues the babies, Maggie is first seen wearing a bow. Then, when she's next seen, it's gone. When we see her one more time, the bow is back.
Continuity mistake: When Millhouse is being chased outside by a sun ray, the ray burns the pavement. It then turns around, and the pavement is now no longer burned. (00:01:40)
Continuity mistake: In the beginning, at the hospital, Maggie's bow changes from red to blue when Marge says "Why are you doing that?"
Continuity mistake: In the dinner scene at the end, Maggie is not wearing a bow, but when everyone tells Moe "Merry Christmas", she has her bow on.
Continuity mistake: During the revival meeting, Ruth Powers has a bandana on in some shots, and doesn't have it in other shots.
Continuity mistake: Homer was wearing a knit cap when the bucket fell on his head, but didn't have it when the bucket was removed. And since he was wearing a knit cap when the bucket fell on his head, his hair shouldn't have been ripped off when the bucket was removed.
Continuity mistake: When the doctor asks him if he has a moment and Mr.Burns looks at his watch but by the next scene it is gone.
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