When my son was about 7, I booked him on a CPT-LHR flight as a UM. Nearer the time, I booked myself on the same flight but forgot to tell the airline to cancel the UM request.
When we checked in, the check in agent told me I couldn't sit next to him because he was a UM and only the person who had made the UM booking could cancel it and had to do so by phoning the call centre. I explained that I was the person who had made the booking ....... it took the intervention of a supervisor to get this done. Seats were changed, and when we got on board I was told by a FA (who obviously hadn't had the updated information) that I had to move. It was all quite good-humoured but had the makings of a farce!