Discovered this when I went to do bwl for a quest, when you fight Nefarian he casts a debuff on priests which causes their heals to put a debuff called
Corrupted Healing - Spell - World of Warcraft
it's suppose to only do 44-56 damage per second, but the debuff scales with spell power, so my priest has about 3.4k with inner fire and it ticks for 3k+ per second and it can stack the more you heal people. The debuff also affects people you heal outside of your raid.
The exploit is pretty much, as you kill Nefarian, when he calls the class Priest, hearth, and when you do quickly heal as many people as possible (direct healing spells only, have not tested with empowered renew) and they will all die taking 3k ticks per second.
You can pretty much only do this once, but it's hilarious when you kill everyone in Orgrimmar or even Dalaran.
SS Imageshack - wowscrnshot081710145428.jpg
Sorry if this is a repost, i searched Corrupted Healing, and could not find a topic that had to do with it.