I recall an incident a few years ago at LHR when the A/C was ready to depart and then the final passenger arrived at the gate.
Check in counts and boarding counts etc all looked OK so ready to go.
On investigation it was found that a one in nn chance had happened.
Passenger A arrived at the gate and his seat no was checked, but it was not noticed was for another flight, and he boarded.
Passenger B, with the correct, for that flight, boarding card, then arrived and just happened to have the same seat number.
CC please keep up the good work and keep checking.