Marge vs. the Monorail - S4-E12
Trivia: Conan O'Brien, who wrote this episode, did a live rendition of "The Monorail Song" at the Hollywood Bowl in 2014 with the Hollywood Gay Men's Choir and Hank Azaria. He dedicated the performance to Phil Hartman.
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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