Campaign ad narrator: Mayor Quimby supports revolving door prisons. Mayor Quimby even released Sideshow Bob, a man twice convicted of attempted murder. Can you trust a man like Mayor Quimby? Vote Sideshow Bob for Mayor.
[Bart is in class discreetly listening to an incarcerated Sideshow Bob on a talk radio show.]
Birch Barlow: I am going to make it my mission to see that our friend Bob is set free.
Bart: No!
Ms. Krabappel: Well, despite Bart's objections, the people of South Africa can now vote in free democratic elections.
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