Thorns
Today I learned the hard way that the roadsides of Eastern Oregon are peppered with thorn bushes and that their thorns lie hidden on the rough pavement, waiting patiently to embed themselves by the handful into innocent bicycle tires and send bicyclists pedaling furiously home, air hissing all the way, only to have to walk the last mile home because the tire's gone totally flat.
I'ma upgrade my tires. Pronto.