This exploit only works if you are a worgen. I've duplicated this a few times over the past few years, and it still works as of the arrival of Mists.
When you do the following, you will glitch the game while in BGs or Random Dungeons.
In BGs you cannot afk out of the battleground. If you do attempt to /afk out, the following happens:
*** You stay in the battleground
*** You get the deserter debuff
*** You gain NO experience
*** The game thinks you left, so..
***** You are removed from the scoreboard
***** The game adds a player to replace you, even though you are still there
In other words, on top of gaining no experience, for every one of you who does this, the game gives your team a bigger advantage. If 1 worgen does it, a WSG becomes 11v10. If 3 worgen do it, a WSG becomes 13v10, and so on. The only downside is when the BG is over, you cannot exit the BG and are forced to Alt-F4 out of the game.
In Random Dungeons, you cannot be removed from the dungeon. If the party attempts to vote-kick you, you will leave the party, but you will remain in the dungeon. I'm not 100% sure, but given how the game reacts in a BG, I figure the game will likely attempt to replace you, resulting in 6 players being in the dungeon at the same time.
Steps to reproduce the exploit:
First, you must create a new worgen character.
Next, Since there will be no vendors or mailboxes available while you are in the exploit stage, you must send this new character all the gear he will need that he cannot get from random dungeons.
Next, Complete all of the worgen starting zone quests up to and including the quest where you destroy the zepplin.
When you have destroyed the zepplin and completed that quest, you will then be sent to talk to the Night Elf on the boat. DO NOT TALK TO THE NIGHT ELF!
As long as you do not talk to the night elf, you will be able to exploit this fun bug. Simply queue for a BG or random dungeon, and have fun!
This exploit has existed for a very long time and was reported to blizz an equally long time ago. It still has not been fixed. I doubt it ever will be.