Might that have something to do with what you just described in the other Desktop thread (Post #39836)? It might still believe that a second Desktop exists.
That post dealt with jaybass's issue...had nothing to do with me.
Somehow I construed your comments
I booted into my Snowy partition and emailed /Volumes/HDz/System/Library/PreferencePanes/DesktopScreenEffectsPref.prefPane to myself, and I received this, with the icon correctly labeled DesktopScreenEffectsPref.prefPane. (Clicking on the image enlarges it.)
The icon in the email is the light switch you described, and when I dragged it to my desktop it resolved into the correct icon...DesktopScreenEffectsPref.prefPane.
to mean that you actually introduced something new to your system. My response just alluded to that. Sorry for the misinterpretation.
