- When Coach is sold out in advance. On planes where the Coach seating is full, you can usually get a seat only if you pay full-fare (Class Y). Often, the price of a full-fare Coach seat is higher than that of a discounted First Class seat.
You might assume that other passengers would have already taken these discounted seats, but there's a twist: many business travelers are forbidden to book First Class tickets (even if they're cheaper). That's no guarantee, because some airlines automatically upgrade full-fare Coach passengers at the time of ticketing, but you can potentially save money and enjoy a more relaxing trip by taking a minute or two to check.
- When restricted Coach awards are gone. The legacy carriers have two types of award fares. Restricted awards are subject to capacity controls (i.e. only a certain number of seats on a given flight are eligible). Unrestricted awards are good for any seat but require twice as many miles.
What you may not realize is that the miles required for an restricted First Class award are usually the same or even fewer than those needed for an unrestricted Coach award. Most people, though, don't bother to look; once they see that restricted Coach isn't an option, they'll be looking at unrestricted Coach. That can mean First Class seats up for grabs even within a week of the flight.
Some airlines make it easier to grab these seats than others. On Continental, for instance, both types of awards are shown for both classes of service whenever you search for an award ticket. That means you're less likely to find them hiding. United, on the other hand, requires a separate search for each type and each class of service--too much hassle for most people.
So, take the time to look. But don't wait too long. As many as 72 hours before a flight, airlines start processing standard upgrades for their elite members and assign those unsold First Class seats to them on the basis of fare code and seniority. Once that happens, there won't be any more discounted First Class fares or restricted awards available.