Whenever someone says they'll do something, someone else comes back with "You won't!" It's almost like a dare. If the person does said thing, the "You won't"-er usually comes back with something like "OK, so maybe you will."
Commonly used by the surf-bum kids who pioneered such gems of slang like "Dude" and "Sweet".
Person1: Man, every time my math teacher gives me homework, I want to pull a Columbine on her!
Person2: You won't!
(Person1 does. Person2 is arrested as an accessory.)
Chemistry Teacher: Now, Hydrogen gas explodes very easily. I could show you a demonstration, but I don't have time.
Pyromaniac Class: You won't! You won't!
Chemistry Teacher: Well, I GUESS we could squeeze it in...