Hi!
I found 2 bug:
1) http://pastebin.com/kY0R6ck3
I opened the Programable RedNet Controller block GUI while my friend broke that block and the client crashed.
2)http://pastebin.com/b1Xnf8ZZ
Just opened a singleplayer world to LAN and a friend connected and crash again. (Restart and works fine)