Ok, I'm in Krokotopia (not where I am now) and I just defeat an enemy with the exact amount of Attack Points needed to defeat him. Suddenly, my computer freezes for a couple seconds. Next thing I know, I'm fighting some invisible monster with 0 Health Points! Oh what an intense fight that was.
Lots. It's an old bug. There's some kind of conflict caused by a 1 point per round automatic regen rate on enemies. Players might have it too -- I don't know. Why such a regen is in the system to begin with, I can't fathom, but I'm guessing it causes enough of a complication to make the bug not worth fixing.
It did to me. I was in the entrance to the arena fighting the spiders. I hit one of them with a spell then it died. Then in the next round it was still there. All I could see was its legs cuz spiders curl their legs when they die. And its still had health. It was weird. But it died after a few of my spells.
Actually, it happens to me, like, every day! I think it's because everything gains 1 HP for some reason every round, so if you kill it with the exact amount of HP, it sort of comes back to life? Lol, or it could just be a weird bug. But the first time it happened to me, and this invisible space was attacking me, I sort of freaked out. xD
What happened to the monster is what I like to call "Natural Healing". Every round, everybody on the playing field has a chance to get 0-3 health points. The monster you were fighting must be below 0 hit points by 3 when the round starts or he will not be dead. This benefit is given to you as well. Watch your health very carefully before and after the beginning of each new turn / round and you will see your health slowly going up. This is also not exclusive to PVE. It also occurs in PVP! So make sure that if you are going to make a kill that it will take them WAY below that zero mark.
It happened to me when I was helping a friend kill Kraken (I'm on MB by the way), he had 0 health. Then my other friend cast a Meteor Strike spell and he was down, but if he has 0 health just hit him with any old spell.