When I played version 1.0 and saved a world in my .minecraft for 1.7.9, I thought it would be fine since in the World's subtitle it said: "Needs to be converted!" So it did. (Note: it was originally played in 1.7.9 as a superflat world, but I used the same map in 1.0)

I expected the map to be normal, but it wasn't. It was half flat, and half mountainous! I have no idea why this happened and I never did something like this before.

To reproduce:
1. Make a superflat world with preset "Overworld" in 1.7.9
2. Save, and play the same map in 1.0 (note that 1.0 has no superflat!)
3. Save the map, and then play it again in 1.7.9 and get bizarre things.