On my macbook pro retina, I use a 3-finger swipe up gesture to bring up mission control. I then decide to click back on to the same app i was using, which happens to have my cursor hovering over it, so I simply left click. Nothing happens, because after opening mission control, the cursor must be moved in order for it to highlight an app window, even if it's already hovering over it. It's just one of those little things that's annoying, it's happened quite a few times to me. Sometimes, you just want to view mission control, and then close it again. It's instinctive to either 3-finger swipe back down OR to just click the app window you were using, and if you notice your cursor is already hovering over the app window, it's instinctive to just click (which, as things are currently, doesn't work).