Continuity mistake: Barney is simultaneously seen at the wrestling stadium watching the fight, and at Moe's.
Continuity mistake: While Edna is holding the "Goddess Bar", her shirt sleeves are pink like her pyjamas, but in the next shot, she is wearing her usual green outfit.
Continuity mistake: The size of the smiley face on Krabappel's mug keeps changing between shots. (00:04:50)
Continuity mistake: Bob duct tapes Bart's mouth before telling him his story, but the duct tape disappears when the story finishes. (00:12:30 - 00:14:10)
Once Upon a Time in Springfield - S21-E10
Continuity mistake: The merchandise Krusty asks the kids to hand back doesn't appear in the kids' hands until the merchandise is taken off them. (00:01:55)
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.
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