Crash when trying to load a world. Does not happen 100% of the time.

Put the summary of the bug you're having here

What I expected to happen was...:
I was going to start my world on snapshot 13w04a and expected it to open quickly.

What actually happened was...:
The game crashed after being frozen on the Minecraft start screen for about 10 to 15 seconds.

Steps to Reproduce:
1. Try opening a world
2. After about 10 seconds if your world does not load it should crash.