I don't know if this has happened before, or even if this is the right place to talk about it. But I figured it might be able to happen to others and believed a warning was deserved. I booted up a creative world I have used since the day of the addition of Creative Mode today. The map is normally populated with nearly 2 years worth of work ranging from a maze that spans half the map and a Let's Play hub that some friends and I use for recording. Today it decided that, all of its life, it had wanted to be an ocean with a single mushroom isle and thus somehow changed to said configuration.
(In short: a creative super-flat world somehow became a standard world with random seed. The map has remained on the same USB stick all of its life and the map is used almost daily. I believe it is beyond saving, and therefore will not ask for help. But in the event that this can happen to others I felt the need to post. Has this happened to anyone else?