Bart Gets an Elephant - S5-E17
Homer: Look at these bills: chains for elephant. Shots for elephant. Over-sized decorative poncho?
Bart: Technically it's for a giraffe, but I think I can let it out a little.
Homer: Well these bills will have to be paid out of your allowance.
Bart: You'll have to raise my allowance to about a thousand dollars a week.
Homer: Then that's what I'll do, smart guy.
Bart Gets an Elephant - S5-E17
Homer: Son, when you participate in sporting events, it's not whether you win or lose, it's how drunk you get.
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