@hamilton,
Strangely, I've been thinking about this very same topic these past few months.
Here's a hypothetical scenario:
I. Congress pretty much guts NASA. Virtually privatizes space exploration and missions.
II. The asteroid monitoring system basically falls apart due to neglect and lack of funds. After all major corporations can see no short term profits (let alone pure unadulaterated profits to fill their collective coffers).
III. An approaching and uncharted asteroid the size of Manhattan goes undetected. Slams into Topeka, Kansas. The ensuing blast and subsequent nuclear winter places humanity on the extinct species list.
I leave the lesson learned from this inevitable scenario blank.