Whenever I start up the world, Java doesn't respond.

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Whenever I start up the world, Java doesn't respond.
I thought Minecraft would just let me play like it did in 1.5.2, but instead it crashed.
The game either crashes while loading or immediately after it starts up.

What I expected to happen was...:
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What actually happened was...:
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Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...