Ever wonder why we had the stat squish in WoD? Wonder no longer! http://wowpedia.org/Stat_squish#The_issue
The basic gist of it is as follows:
Additionally, bosses have run into issues where their maximum health has grown too high. WoW stores health values in signed 32-bit integers, which have a maximum value of 2^31 - 1, or 2,147,483,647. Ra-den, the heroic-only boss of Throne of Thunder, starts at roughly 1,500,000,000 health in 25-player mode. As part of the fight, if players make mistakes his health could increase to the point where it would overflow. In order to avoid repeating the problem, Garrosh Hellscream must heal (from 10% to full) several times with a smaller maximum health pool than would otherwise be necessary.