There is an exploit going around that will allow players to become invincible when they are stealth with agro. Here is step by step how you can actually accomplish this in the game.
1) First of all make sure you have a character that can become stealth in the game.
2) Now go into an instance in the game.
3) Find a mob in the game and get it to agro on you.
4) Now turn your character into stealth mode.
5) When you are in stealth mode and have agro on you in an instance it will bug out, and no mobs can hurt you as long as you are in stealth.
Usefulness:
1) This is great if you want to lead some players through a dungeon in the game easily. All you need to do is take your toon and get all the mobs to become agroed on you. Then take all those mobs to a certain point on the instance so its safe for everyone else to easily get to the end of the instance.
2) This can be used to help you solo instance bosses. Just use this get the mobs all on you. Lure them into an odd part of the map where they cant really move around and then go for the instance boss or maybe try to jump over an object the mobs cant normally do.
3) Other bugs with this havent been fully discovered but im sure you can play around with this to grief players, help get platinum in the game, and help you gain hard to find items.

