Put the summary of the bug you're having here
What I expected to happen was...:
Describe what you thought should happen here
What actually happened was...:
Describe what happened here
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...
This is a technical support issue; this site is for bug reports only. For technical support please use the Mojang Support Center.
However, I'm pretty sure this is an issue with Java 7 under Mac OS X - downgrade to Java 6. Please see the support section for any further queries.