This morning I observed a cyclist do two stupid things. The first was taking off across a junction, against a red light, while traffic coming from the left still had a green light and almost mowed the cyclist down. The cyclist was following some other idiot who did the same thing, but managed to clear the junction ahead of the approaching cars. Riding east on Folsom Street, the Sixth Street junction has a light that turns red for traffic to the right, and a few seconds later turns red for traffic from the left, before a green light comes. It is the same at Folsom and Second Street, and I have seen cyclists cross against a red light and even shout at cars who had a green light that they were obstructing. Really dumb.
The second dumb thing this cyclist did was pull out of the bicycle lane, into a normal traffic lane, without looking behind. A vehicle in the normal traffic lane had to brake to avoid hitting the cyclist. Sure there was something blocking the bicycle lane for everyone to see, but you should not assume that traffic is saving you a space to pull around anything in your way.
You don't have to be a dumbass to get hurt while riding a bicycle in San Francisco, but it helps.