I have an issue with my taskbar this morning it wouldn't display on the desktop. I pull out xterm and did a xftaskbar4 and just return the prompt but no major thing happened. Also on the XFCE desktop control panel the panel configuration wouldn't display. I havent had done anything odd as far as I know.
I know that on my last restart I had Krita n screen and it had become unresponsive and when I boot back up I saw krita on 'top' I kill it but it seems the program was not on screen either.
This might be as far as I got on the clue hunt.
Do you have Xfce 4.4 or 4.2?
I have 4.4.1