Twice in the same world, (both times at level 27 I believe), I log into a world and I'm at the sky limit above the original spawn point. I then proceed to fall and die. The second time this happened, I tried to launch the world, but it froze and I closed the application. When I restarted the application and launched the world, I had no inventory  and no levels. When I went to the world spawn, all my stuff was there, scattered. The first time, I actually watched myself fall past the clouds and hit the ground.