The mob called "Unyielding Sorcerer" in Hellfire Peninsula causes server disconnects when it casts Frost Nova on players. Not sure if the debuff or the spell causes the DCs. It only just started happening, because some players have completed the quest to kill those mobs.
Probably best to avoid that area. And Frost Nova in general doesn't seem to cause disconnects -- some players have tested it in PVP and had no immediate issues.