cross-thread UI updating?

Cross-Thread UI updating is bad and will throw exceptions if you violate a cross-thread action.  To workaround this, you need to invoke the control you wish to update.

if (somecontrol.InvokeRequired)
{
     somecontrol.Invoke( /* ... */ );
}
else
{
     // some code
}

Check out the Control.Invoke method on MSDN.