Had the RD-180 engine shut one second earlier than it did, Centaur would not have been able to compensate and Cygnus would have ended up in a lower orbit. With Shutdown another 1.3 seconds earlier, there would not have been a mission for Cygnus as perigee would have remained within the atmosphere, even when using all of Centaur’s performance.
SpaceFlight101 breaks down Atlas’s launch from last week in amazing detail. It shows how little margin you get when you hurl things towards space. Tip of the hat to Centaur, which did an admirable job saving the mission, and good luck to ULA’s teams for the next step — figuring out what went wrong and fixing it and make Atlas V even more reliable.