I have seen this topic before, but here is what happened to me. After being out of the usa for over a month , imagine the stress when I checked in and I was unable to get a boarding pass for the lhr-mia segment. Website said unable to print in progress, and to pick it up at the airport. When I checked in, the agent said "sometimes it just wont do that". Her words exactly. The reason was as seen before on flyertalk that both of us got swiped for bomb making residue at the gate before we boarded that segment. Not sure why, but no big deal. Made it home on time and had an uneventful flight with decent service.