When I play a pre-loaded world it crashes.

Put the summary of the bug you're having here

What I expected to happen was...:
I would be able to play my world as normal.

What actually happened was...:
Minecraft crashed when I clicked to play a world.

Steps to Reproduce:
1. Load Minecraft 1.6.2
2. Play a world that was loaded in 1.6.1.
3. As it loads the world the Minecraft launcer crashes.