Why does adding a Desktop suddenly gives me a new wallpaper? I'm on the Mountain Lion 10.8.5 and suddenly find that when I add a Desktop in Mission Control (by option clicking the upper right corner) the new Desktop gets the basic OSX  milkyway wallpaper instead of the wallpaper of the current Desktop.

This is what I expect when I add a new Desktop (same wallpaper):

This is what I get (different wallpaper):
  

This was never the case, and just now this behaviour changed. I have a second monitor attached through the MiniDisplay to VGA adapter and on that display adding a Desktop works as it should (that's where the first example is from). Only the MacBook Pro screen is showing this behaviour.

What can be causing this? How can I fix this?
By default Mission Control uses the same wallpaper as Desktop 1. I assume this is a bug with external monitors where it uses the default galaxy image instead. While not what you are looking for, you can always move the System Preferences window to that desktop/display and manually change the desktop image.