For those of you who've played through Act 2, you will know what the bandit quest is. You'll know that you can choose to either "Help" the bandit, or "Kill" the bandit. What you may not know though is that if you choose to do the opposite of what the group is doing (almost always this means you are choosing to "Help" the bandit), it will make the bandits friendly to you and the other people in your party turn hostile.
That's right, hostile. In Hardcore. And killing them sends them to default. So if you join up with a high level character, you can literally 1 shot all the people doing it.
Some players may be wary of this already and will try to teleport out if they see you've selected to help the bandit. One way I can think to get around this is to be the last person to select whether you are going to help or not. Nobody will even notice you selected help if you are the last person until their face gets melted. If you've already finished the quest then it automatically selects the option you chose to finish it with (which means if you killed all 3 bandits that difficulty, you can not do this grief in that difficulty). In order to make sure you go last again, I suggest you have a friend who can look at the quest window and tell you when everybody but you and him have selected. Then just have him leave the area and you can start the quest. You have to do this because as soon as you open the window to view who voted to do what it automatically will select "Help".
Enjoy raging somebody.
Edit: This was moved to exploits, though it isn't really exploitative (it's a system built into the game and, as far as I know, is intended). But wherever the mods want to have it I suppose is fine.