Mothers and Other Strangers - S33-E9
Continuity mistake: A teenage Homer and his father travel to Utah to look for Homer's fugitive mother, whom they find. This completely contradicts the episode Mother Simpson, where Homer didn't reconnect with his mom until he was in his late thirties, and was told all his life by his father that she was dead, even believing that the grave of Walt Whitman belonged to his mother.
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