Report a bug found on Fedora Gnome 3.6 18. Whichever way you decide to automatically change your desktop wallpaper (e.g. Backslide or with wallpapoz) suddenly at one point new workspaces appear black, new Windows are opened menus, pop-up Windows are black, transparent opens Terminal and so on.
 the problem seems to be due to the fact that while picture_uri is updated correctly, not as happens for picture_filnename that does not change and it sends in tilt the thumbnail generator. I have created a script that takes an image at a time and copy it to a folder in your home with an address equal to picture_filename and picture_uri and the problem doesn't seem to recur. I also created a script that copies the image thumnails/normal folder and/home/user/wallpaper folder to prevent possible blockages in the generation of thumbnails (for safety). In only very rare cases (at most once in hours of use) in which the phenomenon recurs, there is always a defective formation of the thumbnail of the image (as you see in the thumbnail the image in half and the rest appears a single grey stain).