It's in their contracts of carriage: When you buy an airfare airlines do not "guarantee" a particular seat, schedule, or flight. And airlines can be pretty persnickety about who they fly and where they seat them. Back when I used to work for Eastern Air Lines, an employee flying on a company ticket had to wear a suit and tie in order to be accepted for transportation. We always flew in first class if there were seats, but one day I showed up at the check in desk with a nice suit but no tie and I was bumped back to economy. "The way you're dressed, you don't deserve to fly at all!" sniped the ticket agent.
But it's not just airline employee dress codes that can get you booted back to coach—or off the plane entirely.
1. The Air Marshall needs your seat
These guys still fly to protect you, and they're sometimes seated without prior warning. If one of them shows up and needs your seat, you'll either be bumped off the flight entirely or reassigned. And you won't even get an explanation (obviously, the authorities don't want you to blab that there's an air marshal on board).
2. You messed with a crewmember
When Bobby Abtahi, a Dallas attorney, was rushing through one of those large revolving doors at New York's LaGuardia Airport, he supposedly "cut off" a Virgin America flight attendant who was attempting to enter at the same time. Abtahi was booted off his flight after the flight attendant complained to the captain flying his plane. At least Virgin later apologized for the "misunderstanding" and refunded his fare.
3. You—or your child—won't listen to crewmember instructions
Actually, if you do anything to annoy cabin crew or fail to obey their instructions and ignore lighted placard signs, you risk being booted. Surely you've seen these outraged families making the news in recent years. In one such case, a mother and her toddler were invited to deplane when the toddler repeatedly refused to sit down and fasten his seatbelt for take off.
4. The airline oversells or double-books a seat
Even in this era of sophisticated computer systems—or perhaps because of them—airlines oversell flights and even, on rare occasions, sell the same seat to two people. You're now entitled to up to $1,350 in cash compensation if you're "bumped" (do not accept a travel voucher!) depending on the reason for the bump, the length of any delay caused and the price of your ticket. Back in 2013, Delta was fined $750,000 for, among other bumping violations, not informing passengers that they're entitled to cash compensation rather than a travel voucher, and American has paid fines for similar infractions.
5. You're not properly dressed
Again, this is up to cabin crew, but if you wear a T-shirt with offensive wording, are exposing too much flesh, or are otherwise provocatively dressed, flight attendants and cockpit crew have been known to kick people off the plane or refuse boarding. Spirit Airlines once deplaned a passenger for refusing to pull up his baggy, underwear-revealing pants, and Southwest has been known to refuse passage to passengers whose clothes reveal too much cleavage or other body parts. Thankfully, airlines no longer require employees to wear suits and ties when flying on company passes, but they'll take it out on you if they don't approve of your fashion sense.
6. You’ve been over-served
If you’re obviously inebriated or otherwise impaired, don’t expect to fly. You won’t be breathalyzed so it’s entirely up to crew member discretion.