Hell, I've been waiting for 99942 Apophis to swing by us since Stargate was still on the air. Still got five years left on that one, but 2004 when that was first called out as a concern was the first time I really started contemplating the idea of the actual end of the world, in a bang not a whimper.

Sure, Y2K was supposed to cause some chaos, and 2012 was fun from a "what if magic is real" sort of angle, but everything else has been a gradual dawning realization that the world as we know it is probably going to be gone in my children's lifetimes -- not over yet, but profoundly changed, more difficult, the slow closing of the book on a golden age for humanity we didn't fully appreciate while we were in it.