A great video is making the rounds right now of an exceptional player using some unintended consequences to solo multiple bosses in Mogu'shan Vaults -- and as a protection-specced paladin no less! He apparently downed Stone Guard, Feng, and Elegon before Blizzard caught wind of the escapade. You can see a video of the Elegon kill above.
How he pulled this off was by taking advantage of a series of design choices that Blizzard made, combining them all into a hilarious example of the law of unintended consequences in action.
To reach a decent item level, the paladin equipped various bind on equip rares that drop in Mists zones which all had a required character level of only 80. And because he was only level 80, he was avoiding the combat ratings drop-offs for levels 81 and 86 that a normal character wearing those pieces would be subjected too. This would allow him to get an insane amount of secondary stats like haste (I've seen that he had 95%!) and mastery (88%, apparently).
Watching the video, it's crazy to see the numbers that he was throwing out. With amount of damage he was taking, the paladin was easily rocking between 2 and 2.5 million attack power from Vengeance. And because Vengeance did not have a cap, there was really no upper limit on how much attack power could be generated, other than how much damage the boss could deal out.
As a result, this was making every Seal of Insight tick heal for at least twice his maximum HP, and Sacred Shield was fully absorbing any attacks he took. Likewise, it looks like with all the haste he had, the actual absorb effect from Sacred Shield was constantly up and constantly being refreshed. Not to mention the damage reduction boost his Shield of the Righteous was getting from all that mastery.
Perhaps the most important cog in the whole enterprise was the use of a bug in the code with Shield of the Righteous. There is a lot more than I care to detail (because it seems to be an overly clever use of mechanics) but one can game SoTR in a way that would result in a ton of Vengeance and, combined with the paladin's obscene mastery numbers, he was getting Vengeance numbers that shouldn't normally be possible. Uncapped -- of course -- thanks to a pre-Mists change to Vengeance.
Since this first appeared a few days, reports have surfaced that Blizzard appears to have responded by nerfing Vengeance with a hotfix, capping it at max health. They seem to have no plans to go after the 409 iLevel gear that aided and abetted this caper. Action is sure to come on the Vengeance calculation bug.