Continuity mistake: During Tom Savini's presentation, the comic book seller eats a kind of cookie. Fake blood then start to flows from his mouth and he takes his t-shirt to wipes it. There are no stains on it after that.
Continuity mistake: When Comic Book Guy turns off the water hydrant, the two boys in purple and blue trunks change position between shots.
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