Happens to me sometimes also. I'd be going fine and well with my fullscreen apps and swiping between each and then when I swipe to desktop, my background is blocked (by the grey background), and when I try to use mission control (4 finger swipe up) in a full screen app, it goes up and disappears leaving the gray background that blocks desktop... when gray background blocks desktop, on Desktop I can only see the upper most bit of my actual desktop (whatever image) behind the task bar (? is that what it's called? with File Edit View etc) when I have it on translucent. I would like to know why that happens! Restarting/Logging out and then logging in gets things back to normal though...
http://flic.kr/p/aN2Mvr