Minecraft 1.6.1 keeps crashing

Put the summary of the bug you're having here

What I expected to happen was...:
For Minecraft to work smoothly

What actually happened was...:
Minecraft crashed

Steps to Reproduce:
1. open the launcher
2. put in user/password
3. click login
4. click single player
5. select a world
6.play world
7. Minecraft will crash