I created a large structure in the nether, left the nether and logged off. Logged on Later and entire nether has reset (to an earlier save?) and the Structure was completely gone and most of the nether had regenerated (Although my Inventory and the overworld was the same as I left it with items mined out from the nether that had been reset) . No Idea what caused it, since just logging off and logging in normally does nothing to the save/world.