The very upper left pixel of the screen sometimes just shows a white pixel. When i move a window over that one, it will just return to normal behaviour.
I experienced this happens especially after a new startup of XFCE and i'm not sure whether or not this is a bug in xfwm.
Never seen this - Can you try replacing xfwm4 with another WM and see if that remains?
didn't happen in a heck of a long time, so i close this bug.