I have a netbook with an external, bigger, monitor plugged in. If I uncheck "use this output" for the external monitor and then log out and log into a new xfce session, the desktop switcher "thinks" the screen size is the one for the bigger monitor, as if they were mirrored (what happens is that the window rectangles in the switcher are smaller than they should). Everything else works fine, the monitors are not mirrored. Maybe this is something having to do with the initialization order.