I was making the quest for the wolf pet - with an elven mystic - and i was killing the spiders and, in this process, at the same time i deal the last blow on a spider, the spider hits my elven mystic and the HP bar reached 0, but she - the elven mystic - dont died!
Like quite a lot of the numbers in this game, your HP isn't an integer value, it just gets displayed as such. So, when you see "0 HP", it can mean you're at 0.238... HP (after the spider hit, and still alive), at 0.000... HP (after falling down from the top of the Tower of Insolence, and still alive), or at -0.179... HP (and quite dead; or at least you would be if the game engine wouldn't reset it to 0.00 HP immediately - with you still being quite dead).
I enjoyed the 0hp "undead" glitch that occured many years ago where people that you could not kill were able to still walk around and deal damage to other players..