Needle nose pliers that was broken in half. Right through the center of the tread on a Boston street. Instant flat at low speed. In my Saturn. In my BMW at a good rate of speed, 10 miles west of Buffalo WY, a 5" long sheared steel rod with a nut on the end went right through the LR tire and lodged 1/2 way through the fender liner. Loud bang, instant TPMS light, thank goodness for the run flat sidewall that held up and the car kept straight until I coasted to a stop.