If i load a 1.7 world in 1.6 it crashes

IMPORTANT!!!!! I wanted to put in the affected versions Minecraft 1.6 but it didn't let me
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What I expected to happen was: I would just load the world and play it

What actually happened was: My minecraft crashed

Steps to Reproduce:
1. Create a world in 1.7
2. Load it in 1.6
3. Voila! My minecraft crashes