BBCnews is carrying this harrowing report about a sudden discovery that nearly resulted in a call to the president:

Some scientists believed on 13 January [2004] that a 30m object, later designated 2004 AS1, had a one-in-four chance of hitting the planet within 36 hours.

It could have caused local devastation and the researchers contemplated a call to President Bush before new data finally showed there was no danger.

Turned out the thing was actually 500m wide. Still, in my opinion this showed the system worked, in that we actually spotted the rock before it had passed us by. However, it also showed some problems with the system that need correcting.