Showing up at the check-in desk and finding out you've been downgraded is one of the worst things that can happen when you've booked a business class seat, but it happens. Airlines overbook, although less frequently with business class than in economy.
It can also happen when unforeseen maintenance issues require a change of aircraft with less capacity in business. It can also happen for less palatable reasons.
A local VIP might decide that their executive assistant requires seating in business – and it's left to the airline to decide who gets dumped, and deliver the bad news to flyers.
Airlines have a pecking order that determines who gets booted. Passengers who have paid for their ticket with airline points are more likely to face the chop than those who paid cash. Next, there's the love test. Elite level flyers with heaps of status credits are less likely to lose their seats than those with fewer credits.
If you are bumped the airline should offer compensation, and depending on where it happens that can be a legal requirement. For flights originating within the EU, an airline is required to reimburse the passenger between 30 and 75 per cent of the ticket price, depending on the flight distance.
Elsewhere it's up to the airline to decide what they're going to offer. It's almost impossible to find any mention of downgrades in any airline's conditions of carriage, the contract between airline and customer.
The compensation might be a partial refund in cash, vouchers for future flights or a swag of air miles dumped into your account, particularly in the case of an award booking.
Getting an upgrade
For your best chance of getting an upgrade avoid flights with lots of business travellers. Chances are their premium cabins will be full, or else your fellow travellers will have the kind of loyalty points balance that puts them way ahead of you in the upgrade queue. Avoid flights that depart early or late in the day and go for midday, midweek and Saturdays.
Contrary to popular myth, dressing like George Clooney – or Amal - won't get you an upgrade. But if the cabin you're booked in happens to be oversold and the airline is looking to upgrade, look like you belong and you just might qualify. Wear cargo shorts, thongs or a singlet and your chances are zip, even with a Kanchenjunga of loyalty points.
What won't get you an upgrade? "Because today is my birthday", "because didn't you see me on MasterChef"? "because here's a box of macarons my mother just made for you", "because my dog died and I'm really feeling it", – the staff at the check-in desk have heard it and then some, and it never works.