What could have caused this flat? Today I encountered a puncture. I pulled the tube, ran my fingers around the inside and outside of the tire carefully looking for the offending object, found nothing, so concluded it was probably a pinch flat (I had just hit a pothole).  I installed a new tube, being careful to seat it properly to avoid pinches, and re-inflated.  Within seconds the second tube was flat.  That being my only spare tube and my only CO2 cartridge and it being dark and a very long walk home, I called the sag wagon (aka my wife).

Back home, I removed the tire from the rim, turned it inside out, inspected carefully, and found nothing.  Ditto with the rim and rim tape.  And, I should add, these are virtually new tires and rims with barely a hundred kilometers on them.

When I looked at the second tube that failed, I found this:



Those slices are directly opposite the valve stem, and a careful re-inspection of the tire and wheel at that location revealed nothing.  But as you can see, they're quite large slices.  I might suspect a faulty tube if it were just one slice, but two?  That seems extremely unlikely to be a faulty tube.

I know no one can tell me exactly what happened, so my question is what are the possibilities?  What the heck could have made cuts that large that occurred without even riding on the wheel and which couldn't be felt with my fingers on inspection?
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.