Many travelers think that they are guaranteed a seat on a flight after purchasing an airline ticket or upon checking in and receiving a boarding pass. Unfortunately, they need to think again. While it’s reasonably rare to get “bumped” from a flight, all airlines include in their contracts of carriage provisions clearly stating that they do not fully guarantee a ticketed passenger’s right to a specific seat or flight.

According to a piece in the travel section of USA Today, there are at least six common reasons an airline can use to justify kicking a traveler off of a flight.

Perhaps the most prevalent grounds for getting booted is when a flight is oversold. The overselling of a flight can occur for numerous reasons, and it is always a frustrating experience for those affected. In these cases, bumped passengers are typically entitled to cash compensation. As noted in USA Today, airlines usually offer travel vouchers to those impacted by oversold flights, but smart travelers do not accept these and instead demand the money to which they are likely entitled.

Once in a while a bumped passenger is given a certain explanation for the flight being oversold when in fact the actual reason is that a federal air marshal needs the seat.

Failure to listen to or comply with crewmember instructions are other common ways travelers find themselves unable to travel on their scheduled flights. Airline employees are tasked with passenger safety, and they take their jobs seriously. As a result, they are given much leeway to use their judgment to determine when a passenger has failed to comply with or listen to their instructions. They don’t necessarily have to provide warnings, and they will often remove even mildly insubordinate travelers from their flights. This can even apply to children.

It therefore almost goes without saying that hostile passengers can be booted from flights. If any crewmember feels threatened or even disrespected, they may use that as grounds to remove a passenger.

Ditto for travelers who appear to be intoxicated or impaired in other ways. Drunk passengers frequently get kicked off of their flights.

Airlines can even remove passengers based on dress code. If they determine that somebody’s fashion is patently offensive or violative of reasonable standards, they can refuse to fly that passenger.