Tree House of Horror IV: The Simpson's Halloween Special IV - S5-E5
Trivia: When Marge asks Bart to explain how scary the episode is, she mentions the "War of the Worlds" broadcast on NPR. Narrated by Orson Welles, that radio serial was so chilling, a number of listeners actually believed Earth was under attack.
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.
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