Really deep pits and stupidity....
Yes, stupidity and free will.
the way HP are described (and the way I see them) if a PC simpli jumps out of a cliff or let someone stab him, or do something intentionally to hurt himself, to hell with HP, he is either dying or dead.
To exemplify: one of the best sceenes I DMed so far was in a HARP game, and the a PC desperately trying to prove the truth of his apeals gave an arm to prove it.
NPC: "So I can use my sword to cut off your arm?"
PC: "Gladly" I extend my right arm over the table.
DM: Described the way the NPC swang his sword and the PC said he close his eyes and do not move. The sword hit only the table (cliché I know), as the NPC (and the DM) were impressed by the demonstration of honor and rightfullness.
My player knew he would loose his sword arm, no matter how much HP (concussion hits in HARP), but he did it anyway.
Sometimes we must underestand that the rules are that just to keep things in order not to ruim a plot.