Using the Background panel in GNOME Control Center, I change the wallpaper to the default GNOME animated wallpaper (blue stripes that change through the day). The desktop background switches to a uniform blue color. This happens since I upgraded from Fedora 18 to Fedora 19. It happens all the time on this particular computer.
I upgraded to Fedora 20 and can't reproduce with any of the two default animated backgrounds. The old one has been replaced by a new one, so I guess this issue is obsolete.