A few months ago, I was riding into work when a truck passed me at the bottom of a hill. The truck was towing a trailer which came unhitched at the base of that hill. The free trailer took its own course, which was into the bike lane, and rammed me from behind. I went tumbling into the street. I don't remember hitting my head, but the red curb paint and two crushed spots (front and back) on the helmet explained it all. I escaped with some bad road rash and a mild concussion. It could have been much worse.