Sometimes, when you put the computer in "Boarding Mode" there is a delay before the reader is ready and you get the "Set Not In Boarding Mode" message.

Occasionally, you get a dodgy bar code that can't be read by the scanner either so you check details, and punch it in manually.

As SFU? NFW! says, sometimes we get a rejection because of joyful news for you, although it could also be that the computer has selected you for a random security search (USA Flights) or that there is a discrepancy between the baggage tag numbers entered in the computer and the tag number the guy with the big muscles has tried to scan downstairs at the aeroplane's side.