Hello Guys. (I am a relatively new and non-technical Linux user)
Well this problem might be a bit complicated but it has occurred 3-4 time now.
I am using Kubuntu 12.04 with kde latest ppa (4.8.3)

I have two connection in my house. 1 is wireless and another is a USB modem.
Generally there is no problem with speed or connectivity when using either one of them at one time.
But some time i have to use both of them(like i disconnect my wireless and connect via USB and then disconnect it and use the wireless).
Here where the problem starts.
1)My Wireless indicator in Network Manger Disappears.
2)The task bar freezes
3)Cannot use the GUI for logging out, reboot or shutdown.
4) Other processes already running seem to run fine.
Then i have to manually reboot via command line.

I thought it might be a bug. So can anyone help me in this aspect !!??

I have seen a similar problem in my notebook. You can try this http://lamarque-lvs.blogspot.com.br/201 ... uting.html to keep both connections (wifi and modem) activated at the same time.

What is probably happening is that a plasmoid is not properly disconnecting its network sockets when you disconnect, then it is freezing the whole desktop until the socket times out (it can take more than a minute to happen). Which Plasma NM version do you use?