Lookee what I found in my rear tire today: A nice, shiny finishing nail.
The good news is that I think the tire's okay. It was near-horizontal so I'm very confident it didn't penetrate the full depth of the tread. The tire's not leaking but we'll see... I taped a tire gauge over my ignition switch to remind myself to check the pressure before I go riding next time.
Made me think, though... just what goes into the judgment call on whether a nailed tire is okay? In this case, given the angle of the nail and the amount of tread remaining, it didn't go as deep as the grooves do... so therefore safe IMHO.