I was one of many who had passport renewal problems last summer, and I became worried that I might not be able to take my flight to LHR. I went to the airport to talk to a real United customer service rep face-to-face (not wanting to hazard the overseas call center). She told me that if I was unable to take my scheduled flight due to passport problems, I should call United before midnight and explain. This would keep my ticket valid for a future trip. If I did not report my problem to United, the ticket would vaporize at midnight.
As luck would have it, my passport arrived the day AFTER my scheduled departure, but since I had notified United, I was able to rebook and get squeezed onto the last possible flight that would allow me to attend the conference that was the purpose of the trip.