Although I've probably committed most of the blunders mentioned so far, Sarah's comment recalled a particularly memorable one for me.
The scene: Winter. I'm a stay at home house-husband watching my baby daughter. I am putting pedals on my '48 Drysdale track bike. Nonchalantly spinning the cranks backward, when I space out and allow the a finger of my left hand drift into the chain and chainring. Ever notice how unforgiving fixed gears can be? The cut was ugly, even more so because the gears were not "squeaky clean" at the time. So I' m bleeding and feeling queasy....I don't even want to look at it.
I wrap it in paper towels and run into the snow covered streets outside. I flag down a a car. I ask the unsuspecting soul to take me to the ER. "OK" he sez... "Wait a minute...I gotta get my kid"......so I run in, wrap my daughter, and away we go.
Needless to say, I am now VERY careful when installing pedals on fixes!
New Britain, CT