I had the following experience on my commute home last night:
It was completely dark, my bike has several lights and reflectors. I was riding down a street which is usually one lane in each direction, approaching an intersection with a RTOL, and a red light. I was the first to arrive at the intersection and positioned myself in the centre of the through lane. While waiting, a car coming from the opposite direction came to the intersection with his left turn indicator blinking. There was a teenager riding a bike towards me, on the wrong (my) side of the street, and on the other side of the intersection. Teenager had no lights or reflectors on his bike, no helmet, and was wearing a black hooded sweatshirt. When the light turned green, the car in front of me tried to make his left turn in front of me, which would have resulted in his hitting the teen who was now riding through the intersection at full speed (he never intended to stop for the red) - I yelled "WATCH THE KID" and pointed at the ninja cyclist. Motor vehicle slammed on his brakes, I started to roll, and the teen looked at me, gave me the finger and yelled back "Mind your own f***ing business" - totally clueless that he just came close to becoming a hood ornament on an SUV.
When I related the story to my wife after I got home, she felt that we would be better off with teens like that removed from the gene pool before they got a chance to breed, and I should have just kept quiet.