I think about this often. Every day in rush hour traffic I have to take a number of calculated risks in order to get to work. This usually crops up when making a turn onto a highly congested road. I could easily see a self driving car being too conservative to find a gap and pull out, leading to a furious string of drivers behind it.
That's why the google cars have a little 'aggression' factor which makes them move out a bit without waiting for a completely clear gap.