With a scaled background image, rubberbanding icons on the desktop damages the background. The residual bits of image are unscaled.
This is easiest to reproduce with centered/scaled images larger than the desktop.
We (Red Hat) should really try to fix this before next release.
I'm not sure if this is the same or different from the bug where occasionally the background is not set on the desktop window. The symptoms of not setting the background will usually be that areas with no icons have a solid-color background, while areas near or underneath icons do get the image background drawn. It happens eventually if you sit in background-properties and change the background enough times. I haven't been able to find the cause of the problem yet.
Only critical fixes for GNOME 1.x at this point. Relevant code is entirely different in GNOME 2 so the bug won't apply (though others doubtless will!)