Fairly frequently, when I open a saved world I will discover that the game will have reverted to the original creation conditions - I'll be standing at the original spawn point, usually with an empty inventory or just a blank map in hand, and all progress/changes in the game since the world was created will have been erased.

Also, if I made a back-up of the world when I most recently saved and exited, that back-up will also revert to the original world as originally created conditions. Older back-ups usually work correctly.