Homer Simpson in 'Kidney Trouble' - S10-E8
Marge: Aren't you going to give him the Last Rites?
Reverend Lovejoy: That's Catholic, Marge. You might as well ask me to do a voodoo dance.
Lisa: Dr. Hibbert, I thought you located another kidney for Grampa?
Dr. Hibbert: Larry Hagman took it. He's got five of them now, and three hearts. We didn't want to give them to him, but he overpowered us.
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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