I'm using "Hide All Applications" to clear the deck before I set up for a new context.
Sometimes when Safari is open and gets "hidden" by this action, the window that gets hidden goes into a Zombie state.
It's in the menu bar, but the image of it there is transparent. When I hover over it I see the page title, but there's no image of it there. When I click on it to activate it nothing appears to happen, but if I try to click on what's on the screen nothing responds -- it's as if there's a transparent Safari window over the screen!
When I quit Safari the Zombie goes with it.
It doesn't happen all the time, but when it does it's annoying!