Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Deliberate mistake: In "Clown Without Pity", when the Krusty doll accidentally knocks the head off of Lisa's Malibu Stacy (her doll), Krusty puts it back on but you don't see a crack where the head came off. (00:07:40)
Tree House of Horror III: The Simpson's Halloween Special III - S4-E5
Deliberate mistake: In "Dial Z For Zombies", Homer is shooting zombies with the shotgun at the school. When he shoots the first two he just shoots them twice. Every other zombie he shoots requires him to slide the gun. It doesn't when he shoots the first couple of zombies. (00:19:50)
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