The gate scanner should caught this but when the scanner has a habit of malfunctioning the GA may ignore the error. The backup mechanisms are the seating problem, the destination announcement and the passagner count. My guess the combo of inexperienced passenger and the holiday rush and human frailties all contributed a bit.

It is not that unknown -- I would say once a year I see a near case of this. That is, one caught late in the process -- on board the aircraft.


not sure what airport security has to do with this, it was a gate boarding error.