Well anyone who has done the fight knows its fairly simple, and that there is quite a bit of raid dmg. Every once in a while he spawns either a Sister of Pain or infernal, it changes every time. The sister is easy to handle, but the infernals (3 of them) spawn from a volcano and shoot out in all directions, as well as occasionally jump into the raid and do a hellfire.
*Now, the bug* is that if the tank kites/tanks Jaraxxus as close as he can get to the outter wall of the arena, the volcano can spawn partially inside the wall, which bugs/eliminates all the infernals by spawning them outside of the arena.
This happened twice in the run i had just done, the first time none of the infernals spawned at all, while the second time it happened, one did manage to get through the wall. This gives the ranged dps time to just focus the boss and gets rid of a good bit of raid and tank dmg. I would also like to note this isn't a 100% chance of happening, as we did get one that spawned just slightly outside the wall.