A 30-50 metre-wide asteroid just passed seven times closer to us than the moon, glowing so bright you could see it through a cloud. If it had hit the ocean, it would have tsunamied.
The Sydney Morning Herald says that if it had been headed toward a populated part of the world, we would have had 24 hours to act and evacuate. Sky and Telescope says that it was about twice the altitude of our communications satellites.
To put it into perspective, here's io9's list of scariest asteroid attacks on Earth, not including this one.