it is also possible that the flight was boarded manually - ie the machine that reads the boarding cards at the gate often go down or just dont function well. to save time and avoid delay the flight is boarded by removing the stump, putting it to one side then when its assumed that the flight is correct at gate the sequence numbers are manually input into the system. the chances are that any pax who have boarded the wrong flight will have it picked up by the cabin crew on inspection of boarding card at aircraft door, or, by the gate staff who end up putting the same sequence number in twice which means the machine will reject or highlight it as an error message will appear on the screen.