Continuity mistake: When Mr. Burns realises just how much Grayson is adored by fans, Warburton Parker has multiple bracelets on his right wrist, but in the next shot as Parker tells Burns about the media-driven world, his bracelets have vanished; then after Smithers opens the floor's trapdoor, Parker's bracelets reappear as he falls through the trapdoor.
Continuity mistake: During the episode, Grayson Mathers wears a watch on his wrist that inexplicably vanishes and reappears a few times. First, when Marge is in the school infirmary with Grayson, as he says, "I called Peyton Manning a chicken-fried Frankenstein." Second, when Grayson and Bart tussle after family dinner, and Marge tells them to behave. Third, when Mr. Burns shows up at Grayson's birthday party.
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