The idea that such a tiny rock could cause immediate failure had me thinking about changing my prefered brand of tires. But after Malta Tire installed new rubber I saw that the "pebble" was just one end of this: A perfect tire assasin. My best guess is that the front tire tossed up the rock which then fell in front of the rear tire in a perfect jabbing position. Nasty rock and worse luck. But I always carry two spares on a long trip, and it was about time to change out the rear tires anyway, so just a mild inconvenience in a wonderful bird hunting trip.