i've had that problem, it means that you have the same file/image in use at the same time.
like say for instant, photoshop stuffs up and you can't close it but you can still work with your computer. you open up another photoshop and try to save your worl but you still have the other image in use.
BTW, I think that is it. If that just happened to happen, try Alt Ctrl Delete and then End Task.