Trolley tracks.
I took my wife down with me on the tandem when I tried to cross the tracks at too shallow an angle; right in the middle of San Francisco's Fisherman's Wharf tourist district, right in the middle of the day, right in the middle of the summer tourist season this year. Three nurses on vacation came running to the rescue while we were still splayed out on the street wondering what had happened to us! After a few minutes of recovery we went on our way a bit shaken and sporting some brand new red and oozing skin that had been recently uncovered by forcible removal of the normal skin layer over it. The tandem suffered only a little rash on the rear derailleur.
Brent