Firing a crossbow into his own foot? OW! 
That's one great way to break your own foot. I was part of a group once that needed an extra player for a night or two, so we advertised for one. The one we got was a munchkin.
His character gloried in the name of Stan the Indestructible because he had insanely magical armour that protected him from physical damage, magical damage, and poison. He figured - what does it matter that I'm using a rusty dagger that only does 3 damage, tops, if no-one can hurt me! 
Not surprisingly, the rest of us (especially the DM) hated him. After a couple of hours, the DM finally had enough. Stan was standing at the bottom of a mountain, looking up at the evil wizard on top. With many a mwa-ha-ha and a twirling of moustaches, the evil wizard pressed the plunger on his magical equivalent of TNT (home-brew, don't ask), and collapsed the mountain on top of Stan.
In this home-brew, 0 is still barely conscious and -10 is still dead. So Stan poked his head out from under the rubble, saying "I have -9 HPs! I'm still alive!" 
...and then, as a final act of evilness, the falling evil wizard threw an enraged tomcat at Stan, which clawed his face for damage of 2 HP.
Scratch one munchkin!
@ Nat 1s* Oh no... poor adventurers. 
Poor, silly, cut-in-half adventurers. 
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