Airlines stand to lose a lot of money if they carry the wrong kind of passengers. Bringing someone without the right papers to Britain incurs a fine of £2,000. Plenty of foreign countries impose similar penalties. Which is why you find your passport being studied intently when you leave the UK.
"Check-in staff now have to act as junior immigration officers," says an experienced ground manager for a leading airline at Heathrow. "We have to police everything from passport validity to whether or not an onward or return ticket is required. The agent concerned could face a severe reprimand if they let someone board who is later denied entry to the destination country."
On Sunday morning, Matthew Roberts, a reader from Bristol, arrived at Gatwick in good time for his flight to Faro in Portugal last Sunday morning. He had booked a romantic break with his girlfriend to Algarve coast, staying at a chic boutique hotel.
So meticulous was Mr Roberts's planning, that he even thought to check his passport expiry date: 10 January 2012. Knowing that the immigration authorities in some countries stipulate several months of validity beyond the intended date of departure, he checked with the Foreign Office to see if that could pose a problem. Happily, he learned: "Your passport must be valid for the proposed duration of your stay; you do not need any additional period of validity on your passport beyond this."
British Airways, Monarch and TAP Portugal accept the Foreign Office advice, but easyJet does not. The airline's ground staff said Portugal insists on a minimum of three months' validity. Mr Roberts was barred, and the holiday was over – strangled in a tangle of what, given the corporate colour, must be orange tape.
There are 50 ways to miss a flight, and I have tried many of them. But being barred from an EU country with a passport that has 12 weeks to run is a new one on me. Filipe Silva, director of the Portuguese National Tourist Office in London, was equally baffled by easyJet's three-month minimum rule.
"It makes no sense. Portugal is part of the European Union. If a British citizen has a passport with only one or two months, they are welcome to visit. There are no limitations."
However, as 21st-century travellers must wearily accept, these days the passenger is always wrong. A spokesman for easyJet said: "We understand Mr Roberts's frustration. As far as we can ascertain, our staff at Gatwick made the correct decision according to the information that we and other airlines rely upon. This case has prompted us to make enquiries with the Portuguese authorities to establish if there has been a change in the rules."
The last such change in rules that I can identify took place on 1 January. Not this year, but in 1986 – when Mr Roberts was but one year old and easyJet was still a decade away from being born.
As a "gesture of goodwill", the airline flew the couple out to Portugal two days later. If any airline has treated you in a similar fashion, let me know.
Airlines make more money from not flying passengers than they do from flying passengers. If you travel as booked, they have to pass on some of the fare to the government and the airport. If you don't, they typically keep the whole lot.
Conspiracy theorists insist that is why check-in deadlines have gradually lengthened. Remember the good old days when you could breeze along to the airport half an hour before your flight, and still get on board? Well, one European airline has brought them back.
On Air Malta, the deadline for passengers with baggage to check in at Manchester, Gatwick and Heathrow is 45 minutes. But those are the exception. Passengers with only hand luggage qualify for a30-minute deadline.
And when you fly back from the sunny isle, the half-hour rule applies whatever your circumstances, giving you precious extra time to appreciate Malta's splendid shores and stories (pictured).
When, I wondered, did the airline decide to subtract stress from passengers and hand back some time? I called Air Malta customer services to ask, and was told, firmly: "Passengers should be at the airport at least two hours before departure."