Continuity mistake: When Homer and Ned attempt to escape from their Vegas wives at the buffet. The Vegas Wives are directly in front of them, when Homer runs off he runs in an opposite direction, nowhere near them. When Homer runs out, the Vegas wives are right next to him.
Tree House of Horror IV: The Simpson's Halloween Special IV - S5-E5
Continuity mistake: When Otto hits Hans Moleman's car, it has a number plate on it, which disappears when he crashes.
Tree House of Horror IV: The Simpson's Halloween Special IV - S5-E5
Continuity mistake: Just before Homer is sucked into Hell, he is wearing pyjamas. When he's in Hell, he's wearing his usual day clothes.
Continuity mistake: Milhouse shows Lisa a laptop with e-mails on it. There is also a shot with a list of the e-mails. At no point does anyone click anything, but the next shot shows a different page on the laptop.
Continuity mistake: In the scene where Smithers is showing Burns the bar, Smithers is in his grey suit. When Burns turns to talk to Grizzly Shaun, Smithers is wearing his standard green jacket and bow tie. Afterwards he is back in his grey suit.
Continuity mistake: During the scenes at the site of the mall during the part where they find the Angel, Skinner's hat disappears and reappears.
Continuity mistake: During the wedding you can see in the background Apu and Sanjay but Sanjay's skin is yellow.
Continuity mistake: When Moe is rocking Maggie, the little image of the flower that is on the rocking chair is purple. Then when Moe gets up, the flower is white.
Continuity mistake: When Maggie is opening her presents, the biggest present is pink. Then after Patty and Selma walk away, the biggest present is blue.
Continuity mistake: While in the cafeteria, Principal Skinner asks Milhouse to lower his eyebrows. Behind him, there is a bulletin board that changes from brown to purple.
Continuity mistake: While Lisa is standing in front of the table with her friends in the cafeteria, we see that the trays appear during the first shot, then after the camera zooms in on Lisa's table, the trays disappear. Then during the third shot, Lisa calls Alex over to her table. Look at the table again: the trays have reappeared. (00:06:00)
Continuity mistake: Just before Apu has a flashback, Reverend Lovejoy is sitting to Apu's left. after the flashback he is sitting to Apu's right.
Continuity mistake: When Lisa is getting the answers to her test, the coffee mug changes colors.
Cape Feare - S5-E2
Continuity mistake: When Bob is working out on the weights, he has a tattoo of something on his knuckles, but when he is watching the exercise video, the tattoo dissappears.
Continuity mistake: Moe's back pockets, the pockets on his bum, change position/disappear between shots - before the Italian-American-Mexican stand-off.
The Mook, the Chef, the Wife, and Her Homer - S18-E1
Continuity mistake: When Lisa grabs Ottos tape recorder her necklace is orange but when she gets off the bus it is white.
Continuity mistake: When Lisa arrives at Dr. Nick's, amongst the wheelbarrow line is a Professor Frink (and he's not even first in the cue). However, just a few minutes later, when Lisa ends up in the mud pit, we can see Frink amongst the crowd.
Continuity mistake: In the scene where Maggie swallow' Moe's penny, she first takes out her pacifier to eat it, but in the next shot it's in her mouth again and in the next, she's putting it back in her mouth.
Stop Or My Dog Will Shoot - S18-E20
Continuity mistake: In the scene where the dog chases Snake into a playground, Santa's little helper starts pulling the ropes off the top of the climbing frame, in one shot the dog has two ropes left but when we see Snake the dog chews through one rope and the whole thing falls.
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