crash for 1.7.2 (or bug)

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What I expected to happen was...:
Describe what you thought should happen here
it is a crash or a bug
What actually happened was...:
Describe what happened here
i cant play multiplayer or singleplayer whe i start a world it says :your Minecraft has crashed! and i cant help it
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2.what does it mean?
3.what does it mean?