I finish my Minecraft World off by playing at least 3 HOURS, then I get sleepy, then I SAVE, AND EXIT, THEN... Its says it saves, but It FREEZES right when it says 'Exiting Game' I lost EVERYTHING, and had to go back to the save before that one... PLEASE FIX IT, SO EVERYONE CAN HAVE THERE WORLD BACK )':
http://www.minecraftforum.net/topic/1222871-mcxbla-official-bug-list/