I've already seen a similar situacion.

But it was human error! The screen was flashing red and the operator wasn't looking at it...

This happened on entering the internacional zone in London and was the cleaning lady that asked the operator what was that red light...

In the end, the person had another ticket printed and all was sorted out... Computers and scanners and barcodes and qr codes and everthing else are great in this systems, however, if the users are not even looking at the screen then there's nothing that will work 100% of the times.

Sorry about any error on my english gramar,

Helder