It happened again yesterday riding to the station: rear tire went flat. Pumped up and rode a bit, but it was going flat so fast I stopped at the next street light and changed the tube. The puncture was on the rim side of the tube. The rim tape (velox rim tape) was not perfectly centered, and though I couldn't see any obvious spots where it was missing a spoke hole, I took it off and put it back on, this time taking care to get it perfectly centered.

This may explain both of the flats I had last week. If it happens one more time, I'm replacing the rim strip.