I have found a bug (i think it is) when you are useing the tolls, (do not mather witch one) and stop usesing them and start to use them again they flicks up and dowwn as someone is pressing down the mouse button and try to use the tool. (a little hard to explain but watch the video and you will understand) what i am saying..
The video (MP4) is the best way i could say whats wrong as logs dont say a thing for the moment whats wrong.
i have heard from others when they try to login to an new world Minecraft crash. (mine did to but worked on my second try)