I have a friend that is constantly getting booted from my server and getting a java exception error within 5 seconds of login. We scratched our heads on this for some time and what I discovered was if you go into your client side stats folder .minecraft/stats and delete stats_username_unsent.dat and stats_username_unsent.old in this client side folder and then log into the server it fixes it. The server keeps the info on what you had in your inventory so it will all be as was. I still have no clue as to why this gets fubar but it will fix it. Hope this helps some people out.
C:\Users\Username\AppData\Roaming\.minecraft\stats