Had the same thing happen 2 times, both of which were on United. Didn't both calling, just got to the airport shortly after the flight departed and stood by for the next one. In one case I was able to get on the next flight, in the other case I had to spend sometime convincing the agent to reroute me to OAK since all SFO flights were chock full that day. In both cases I made it to my destination within a few hours of when I should have, so it wasn't an issue.