In the pre-released version of 1.6, the snapshot, after you exit a world, it will no longer be there.

I exited a world that took me a long time to create in the snapshot, but after I exited and went to the "singleplayer" button, my world was no longer there. I expected it to be there, but it was not.

To trigger the bug, you have to
1. Generate a world, with a name or without.
2. Exit the world
3. Go to the "singleplayer" button, and the world you generated will not be there anymore.