Yes, it's unnerving when you're off the standby and cleared on to your flight, but can't see yourself on any list.

I've never had an actual call from a human when this happens, but certainly have been protected automatically, such that when I called in they just told me the new flight info.

In the past UA was always great at this for their 1K's and over the past year, I've been lucky to have been re-routed only once, but still, the system worked.