Remember when frequent flyer tickets used to be free? Ha.
One of the most insidious fees that airlines have imposed recently are atrocious penalties -- forfeiture of all the miles you paid -- for changing or canceling frequent flyer bookings. It's the new "fee" that airlines don't have to list prominently on their websites' fee pages.
Perhaps the worst offender is Virgin Atlantic. Any changes or cancellations incur a $50 fee. If tickets are canceled more than seven days from departure, only 75% of the miles used will be refunded. If canceled within seven days of departure, all miles will be forfeited but no fee will be charged. These outrageous penalties are spelled out in the terms (you know, the mouse print that no one bothers reading, but they should be made impossible to miss in big red warning letters before you fork over your miles).
Next worst: Delta Airlines which now imposes a 100% forfeiture of miles for cancellations within 72 hours of departure. Yep, you read that right. You lose all your miles.
Every airline has its own policy, as this chart shows. For airlines not listed, it's wise to ask your airline's frequent flyer desk before booking. And as always, rules are subject to change without notice.