The Simpsons Christmas Special: Simpsons Roasting on an Open Fire - S1-E1
Homer: [buying a chewtoy for Maggie.] It says it's for dogs, but she can't read.
The Simpsons Christmas Special: Simpsons Roasting on an Open Fire - S1-E1
Marge: You will not be getting a tattoo for Christmas.
Homer: Yeah, if you want one you'll have to pay for it out of your own allowance.
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