$pringfield (or How I Learned to Stop Worrying and Love Legalized Gambling) - S5-E10
Bum: Got any spare change, man?
Grampa: Yes! And you ain't gettin' it! Everybody wants something for nothing. [He walks into the Social Security Office.] I'm old! Gimme gimme gimme!






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