I found a cotter pin
… when riding down 17th Ave this afternoon. It managed to poke through the tire & the tube, then bend and poke a constellation of holes in the other side of the tube. I tried to patch the tube, but it didn’t work very well because the bent legs of the cotter pin tore the tube when I extracted it, and even a fairly large patch wasn’t enough to properly reinforce the tube. (And then I didn’t reseat the tire properly, so about 2 miles down the road it popped off the rim and started rubbing against the DS chainstay – this, oddly enough, was not enough to destroy the replacement tube, so after reseating & reinflating the tire I was able to ride home w/o any further incident.)