On the bike leg of my first Tri after buying a very expensive (to me) carbon fiber rocket, I was passed by a guy on a 1970s Sears Free Spirit 10-speed that creaked with every pedal rotation.

I fell on the sidewalk next to my building because I was trying sidesaddle my bike while holding a sheaf of papers on the handlebars. I dropped the papers, tried to catch them, leaned too far over, and went right over the bars. Because my right foot had clicked into the pedal, the bike landed on top of me.

I pulled the valve out of the tube just before a race by overtightening that little ring nut.