TV Quote from The Simpsons 06x06: Treehouse of Horror V

Homer: I've gone back to the time when dinosaurs weren't just confined to zoos! OK, don't panic -- remember the advice your father gave you on your wedding day.
[In Homer's Memory:]
Grampa: If you ever travel back in time, don't step on anything because even the tiniest change can alter the future in ways you can't imagine.