Mountain Lion's system-wide notification banners can occasionally cover up something that you need to click. Luckily, a quick flick can get rid of them instantly.
Waiting four seconds to click something underneath a banner doesn't seem like a big deal, but it can be surprisingly infuriating. Luckily, just as a right-to-left swipe on an iOS banner will dismiss it immediately, a left-to-right swipe on a Mountain Lion banner will send it off your screen. Just hover your cursor over the banner, and swipe two fingers across your trackpad towards the Notification Center. It also works with one finger on the Magic Mouse.
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