Put the summary of the bug you're having here

What I expected to happen was...:
As I had a similar problem before but then I only got kicked from the server with error messages like Internal Exception: java.oi.OIException: Bad Compressed Data Format and Internal Exception: java.oi.OIException: Bad packed id (numbers like 144, 79, 102 and many more) and several other error messages. As I was trying to fix this with help fomr the Mojang support team I just expected an error message.

What actually happened was...:
What actually happened was that after just a few seconds on the server Minecraft crashed.

Steps to Reproduce:
1. Open minecraft 1.6.1 on a mac
2. Pick any 1.6.1 compitable minecraft server
3. Try to connect to it

(This may be something on my computer, but I doubt it as I have never had issues like that before. I am thinking it may be that somehow minecraft 1.6.1 multiplayer is incompitable with Macs)