http://forum.xfce.org/viewtopic.php?id=5695
What I've noticed of this bug is that the window buttons in each panel seem to forget the "Show windows from all monitors" option as being unchecked. I've noticed that anytime I have to restart X (or gdm), the option behaves as though checked. All I've had to do is open the properties for each instance of Window Buttons and re-check and un-check that option to restore expected behavior.
*** This bug has been marked as a duplicate of bug 7255 ***