Earlier this year I was flying Jakarta-Singapore on Air Asia. Departure was held up about half an hour because we had one too many on board.
It was actually a little funny to watch - cabin crew would go through the cabin doing a head count, then go back to the front and make an announcement that this was flight XXX to Singapore, to make sure Singapore was your destination. This happened several times. Finally they went through checking everyone's boarding pass - found a lady that had gotten on the wrong airplane
