Driving downtown on a relatively busy road after dark and traffic slows quickly. Suddenly I see a cyclist wearing all black and a non-reflective helmet. I think to myself ... idiot!
As I pass him I notice that he has a headlight. Hmmm... helmet, headlight? I slowed down and sure enough there is a blinkie taillight attached to his seatpost. Trouble is it is not turned on.
I didn't want to scare the guy (since I was in my car) so I rolled my windows down and when he got into the lane beside me at the stoplight I yelled: (in my friendliest voice)
"hey buddy -- your tail light is off!"
Later I watched him blink down the road for several blocks.