Here is one more data point for OZ from this past week.
6 months ago we booked JFK-ICN, ICN-NRT, NRT-EWR (UA). I read all of the permanent information on FT before I booked the tickets so I knew to confirm the reservation and get seats and the Asiana PNRs which I did. I also reconfirmed a few days before departure. No problem on JFK-ICN but a few days later I'm standing in line to check in at the Asiana counter and actually telling my wife about this thread and some of the stories that were reported. A few minutes pass and we check in only to find out that the ICN-NRT leg was reserved but never ticketed (according to the agent). I was shocked because I had called to confirm the reservation twice but I never specifically asked "are there ticket numbers for both the JFK-ICN and ICN-NRT legs".
The UA desk didn't open until after our flight was set to leave so the agent let us use the Asiana phone to call UA. After a long hold, the UA agent initially insisted that everything was fine in her system and that the problem was with Asiana. UA said there was no problem with the ticket but eventually figured out the error and resolved it after 1.5 hours on the phone with them. I don't know why it happened but since UA had to fix it from their end I am more inclined to believe that it was there system problem.
Keba