Was playing in my server and starting to test Mystcraft stuff. Made some linking books and placed them in the vanilla chest in the corner. Had my minium stone selected in my hotbar and the game just crashed. Only the client. And now everytime I try to log into my server the client crashes instantly.
Here is the crashlog http://pastebin.com/qXzkdy5n

I suggest next time you have a crash post the "ForgeModLoader-client-0.log". What you've pastebined doesn't really help.

Yeah. Sorry about that. I tried to reproduce the situation in single player, but there it worked fine. But I didn't remember the exact contents of the vanilla chest... I'll post the forge log next time if the same thing happens.

Same crash happened here when i clicked a mystcraft writing desk with a minium stone selected, log here http://pastebin.com/Ur4QWWK5

Yes, this bug destroyed a lot of my worlds. I had it with TE3 though.

Fixed in Forge 727 and higher - just update your Forge to Latest and you should be fine.