The Shulhan Aruch (Orah Haim 560:5) states
אסור לאדם שימלא פיו שחוק בעולם הזה.
It is forbidden to a person that he fill his mouth with laughter in this world.
I don't think I've ever met a person that doesn't laugh. Why don't most people follow this Halacha (i.e. what do they rely on)?