Requirements for this exploit to work: WOW Emu Hacker. Waypoint close to boss and created in such way that you face the boss when you appear. A class that can deal damage instantly. Like mage. A projectile won't work, boss will regenerate before it reaches him. Or you can place a debuff, it will throw him in combat immediately.
One of my pvp partners recently discovered this bug, when we farmed Icecrown citadel. When teleporting to boss, out of the start of the recently reset instance (changed difficulty) my friend noticed, that boss that he teleported to was gaining health in pulses, as if he was damaged and was returning to his positon after loosing all aggro. With quick fingers and /target boss /cast fireblast macro, we managed to "catch" boss, before he gains full health.
So far we have been able to pull it off on Rotface, Blood Queen Lana'thel and Lady Deathwisper (Managed to engage her at 400 health)
It's not a very useful exploit, but it works for us and I haven't seen it posted anywhere else here, so I decided to share. Really makes the job easier, when just two players fight ICC bosses that have huge healthpools.
The server this bug worked on is level 80 20x server.
Or maybe it was posted here before, but I didn't notice it.