This is more of a tip than a exploit. But if you ever bought the WoW atlas it has all the rare monster spawns in it also. There ranked as common rares, rare rares, uber ect.. the rarer the spawn usally the higher the lvl and the spawn time is usally around 78 hours or more. There location can be somewhat random also.
Anyways to the point. If you happen to know were the uber rare ones are and blizzard resets the realms then all monsters are spawned no matter when they were killed. I tested this last patch and previously befor that with a minor update. I killed 3 uber rares that ive never seen befor getting 2 BOE blues and from the 3rd one even a epic krol blade.