ATM it seems that the Y axis is completely ignored in the combat distance check. That makes it possible for npcs to kill you with a sword while standing 3-4 meters below you (for example in the stairs in the laanx dungeon: go down, aggro one of the thugs, run up the stairs until you are above the thug and stop to get killed. doesn't work always, because sometimes the thugs lose interest too easily, but if they don't, you might very well die.)
EDIT: Should this go to the bug tracker?