Okay, so this is a really weird error. Every so often when I trigger the notification centre using the two finger swipe, the machine will freeze. The notification centre can't be dismissed and any action I try to do after that just doesn't work. I can select some things like in the status bar and can even choose to go to login screen but the action does not complete, i.e I can only see a blank screen after that and not the login window.
This is irritating because it does not happen every time and is completely random as far as I can tell.
The only thing I can do after this happens is restart. I'm completely at a loss as to what might cause this as this does not seem to be a common error. A Google search hasn't revealed much.
This has been happening for quite some time and I don't recall installing any specific app after which this started happening. Maybe Better Touch Tool but there's no real connection I can make out.
Retina Macbook Pro on Mountain Lion with all available updates applied.