This problem began when I upgraded to Yosemite. I have two monitors connected to my Mac Pro. Primary monitor uses default wallpaper, and secondary monitor uses a different wallpaper (solid gray). Every time I reboot the system, the secondary monitor reverts to the default wallpaper, and Mission Control shows an additional "desktop" on the primary monitor, with a solid gray wallpaper. After rebooting many times, the list of desktops on the primary monitor grows quite long if I don't delete them manually.
I have read a tip to fix this behavior by disabling this feature in Sys Prefs, but I don't want to do that.