A couple of years ago the handling geniuses for Easyjet at Amsterdam Schiphol marked a passenger as offloaded during the gate boarding pass scan but gave him back his boarding pass. He obviously didn’t know that and boarded with all the rest of us.
After two head counts and a pass of the cabin to check everyone had a boarding pass for this flight - which of course they did - the crew had to resort to walking down the aisle with the passenger manifest doing a roll call like in primary school until they found the extra.
The crew were clearly displeased with the ground muppets.