Treehouse of Horror XV - S16-E1
Continuity mistake: At the beginning when Kang and Kodos's boss comes to have dinner, his stomach explodes leaving a hole, yet in the next shot the hole has vanished. (00:00:40)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Ned saves Moleman, we see where Ned is standing, there is no manhole beneath him yet when he catches Moleman there is one there. (00:02:30)
Treehouse of Horror XV - S16-E1
Continuity mistake: Ned hammers in the For Sale sign; a few shots later, when Homer comes, it is no longer there. (00:03:40)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Lisa goes to see Comic Book Guy, we see that he places the dagger on the table in the centre of the green thing, then when the shot changes and he puts the comic there the dagger is gone. (00:09:45)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Comic Book Guy gets stabbed, we see him reaching for a potion, and in the first shot, its a purple liquid with no label on the bottle, and when the shot changes we see it is a green liquid with a label on the bottle. (00:10:20)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Bart and Lisa go into Mao's, we see above Moe there is a guy wearing a red jacket, then when they're walking past him, he is wearing a light brown jacket. (00:11:00)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Chief Wiggum has been caught, we see him drop his eel pie, then when the shot changes it is not there. (00:13:30)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Chief Wiggum is running for the hot air balloon, you can see in the distance no one is in it, yet when he arrives Professor Frink is in it. (00:13:50)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Professor Frink is showing everyone the giant capsule, in nearly every different shot the colours of the balls change. (00:14:50)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Mr. Burns is lying on the table before the Simpsons enter his body, he is wearing briefs. However, when the Simpsons are inside him, he is wearing boxers. (00:15:55)
Treehouse of Horror XV - S16-E1
Continuity mistake: When Maggie crawls out of the stroller and into the capsule, the stroller has no harness, but when Marge sees the empty stroller, there is a harness.
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