While I do not agree with #teamboardlast...i had a major victory a few weeks ago. I was flying with my wife JFK-TXL on waitlisted GUCs. There was one seat available so I let her have it while I decided to wait by the gate and wait for no shows. After she boarded I noticed that the gate screen still read Reserved seats (unclaimed) = 1 (just like it did before she was upgraded).
While boarding zone 3, I asked the gate agents if first boarded full. They said "Yes" and told me to board. I asked about the remaining seat and they said " 1st checked in full". Then I asked for a second time if it boarded full and that there are not empty seats on the plane. They said yet again that all D1 passengers are on the plane. I asked yet again why the GIDs screen disagrees with them. They said they do not know.
I stepped back again for 5 minutes. Boarding was completed by T-20 and they were calling me on the PA to board. I once again asked about the seat and they annoyingly said "first has boarded full, please get on the plane or we will cancel your seat"
Finally, a light bulb went off on the 2nd agents brain. He whispered to the other gate agents that a checked in DL employee (who was given revenue space in D1 for some reason) has not boarded. By T-5...they finally cleared my D1 upgrade. If it wasn't for the GIDs, I would have never known.