What could have caused this flat?
I patched both tubes yesterday, partially inflated them, and let them sit overnight (without being in a tire).  One of them was flat today, so I tossed it as probably having an unseen slow leak not worth finding.  The other one I remounted and rode 40 miles today without problem.  So apparently either: 1) I somehow damaged the second tube mounting it, or 2) there was something in the tire I never saw or felt on inspection and it fell out without me noticing.