Recently, my world has started to randomly reset my progress while I'm still in the game. If I destroy a few blocks in a chunk and then leave, then re-enter, the blocks will still be there. The inverse is also true. Object deletion/replacement seems to be caused by not staying in chunk long enough for game to register change, but may also be completely random.

Entering/exiting the Nether seems to have even more drastic effects, to the point of reversing "time" itself. Mob positions are changed, objects in chests will be reset and even player inventories are effected, I believe.