When things fail, we gain little by punishing the person who was operating the system. Instead, we must ask, "why did the system allow this failure?" Fix that root cause, because the operator is not the root cause here.
Should be easy enough, UI shows the picture of the guy who scanned the first ticket and system tells operator you go and check his ticket. If you can't find him on the plane he's hiding in the bathroom, call security to get him out.
Given there are thousands of security cameras in an airport, I don't think anyone would mind if one was used to record who got on the plane.
I've lived in a number of countries in the past 10+ years and encountered more than my fair share of edge cases when flying. On the average, the gate agent has to override something 1-2 times per year when I fly internationally.