I just want to say it is not impossible to happen. It has happened to me only 2 or or 3 times in my life, once it was purposely, I think right before September 11, the other time(s, not sure now) was a mistake like happened here. I also recall now that it happened once on a KLM flight from US to Amsterdam. If I had 1500 flights so far that is like 0.2% chance.
The purposeful one was AA from Tyler to Dallas on a paper RTW ticket. The single agent on the ground (ticket agent=gate agent=flight manager) after two hours on the phone to change my ticket talked to the captain and they took me without taking a coupon or checking me in. Amazing but true. It's a long time ago, I hm guess I am still fairly anonymous here and I kept quiet not to jeopardize the jobs on the line. They helped me out for a service shortcoming. Of course I dis not claim the miles :-)
The KLM one was a nightmare. This was more recent. Same thing happened. E Ticket coupon marked no show. No miles. And worst of all no manifest of me leaving us sent to US immigration. Purser came twice to ask my online boarding pass. I still flew, in my assigned upgraded seat. (Op up).
Next entry in USA I was almost arrested for an overstay. I am sure KLM got in trouble. I had proof including the KLM house and fotos but it was a nightmare to get my return ticket working, miles credited, immigration record fixed and compensation. (I think was a service recovery upgrade)