Sometimes you can carry your wallet through the airport X-ray machine, sometimes you can't. Sometimes that trusted-traveler membership speeds you though security, other times, well, sorry. And you might get a surprise check of your driver's license at the boarding gate.

For the past decade, travelers have learned to adapt to a changing array of security requirements—shoes off, liquids restricted, bodies scanned and patted down. Frequent fliers try to adjust, knowing that following...