I searched and didn't find anything but I think it can be duplicated if we figure out how?
Anyways I don't know it was happening but it can make that part of the fight super easy if we can duplicate it.
EDIT: Here's some more info. The way the fight works for people that don't know it. Once Onyxia dies Nefarian flies into the air and the platform lowers to the ground. This takes about 3 seconds and you cant get on top of the pillar till after the platform is all the way down. The glitch/bug/exploit is that the warlock in our raids lava was filling up instantly and he didn't take any dammage whatsoever from the lava.
His latency was higher than other peoples so that's not it and we have no clue how he kept getting yp so easy.
On everyone's screen he was floating up to the top of the pillar.