My panel is set to transparent. The portion of desktop background underneith the panel does not line up with the portion of the background on the desktop. Actually sometimes it is aligned but when you open or close a window and a new taskbar item is put on the panel or removed from it, the background under the panel doesn't stay in sync with the desktop part of the background. It moves a little to the left or the right. On my Kubuntu (KDE 3.5.1) machine I had the same problem until I went into panel configuration and chose the center layout position. The same setting has no effect on my other machine which also has KDE 3.5.1.