Originally Posted by Kevin AA

No, you have it backwards. If the gate agent just scanned A20 and the next person presents A22, fine. A21 can board later or not at all, who cares.

The problem is when A15 was just boarded and then suddenly A21 appears. Step aside and wait for A16-20. It's possible that all five of those boarding pass holders decided to no-show or canceled after checking in, in which case it's now A21's turn. This is incredibly rare and wastes little time.

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