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There are a thousand things to worry about on your way to the airport. But how often do you consider that, even if everything goes right, your airline might not let you board?

About 15,000 passengers were denied their airplane seats involuntarily over the year ending in April 2018, based on the most recent data from the Department of Transportation. During that time period, 630 million passengers boarded a flight on one of the eight major domestic airlines. (See our best and worst U.S. airlines.)

This habit of "bumping" happens because airlines regularly overbook flights to ensure a 100% occupancy by take-off, but when the practice is handled inefficiently, passengers can be left behind. For passengers, this is a major nuisance, and some airlines are more guilty of bumping paying passengers than others.

The five airlines with the worst track records for bumping passengers each served anywhere from 17 million to 158 million people. And for every million who did get on board, the airline denied boarding to at least 24 passengers, some of whom were given alternate transportation or compensation for their troubles. (But what price is worth derailing what might have been a dream vacation?)

Here are the airlines most likely to bump a passenger who's waiting to board their flight.

Spirit Airlines

Spirit Airlines Airbus A319 departs Los Angeles International Airport on June 17, 2017 in Los Angeles, California.
FG/Bauer-Griffin—GC Images

Total boarding: 23,780,619
Passengers denied boarding: 1,846
Passengers denied boarding involuntarily, per million: 78

Spirit Airlines stopped more of its total number of passengers from taking their seats than any other airline in the year beginning April 2017. It had the second lowest number of passengers on our list of eight airlines, and denied boarding to 78 passengers per million boarded. That’s a lot more than the rate of runner-up Frontier, and three times the average rate of 23 passengers per million.

Frontier Airlines

Frontier Airlines passengers Rene Gilstrap, left, and her mother Dawn Ramsden look over options for another flight to North Carolina at Denver International Airport December 20, 2016.
Andy Cross—The Denver Post via Getty Images

Total boarding: 17,432,894
Passengers denied boarding: 964
Passengers denied boarding involuntarily, per million: 55

Frontier denied boarding to just shy of a thousand passengers. The airline is on the low end in terms of passengers served — the lowest of any of the eight airlines, in fact — which means that they turned them away at a higher rate.

Southwest Airlines

A Southwest ticketing line is seen as travelers prepare to board a Boeing 737 Max 8 flown by Southwest Airlines at Baltimore Washington International Airport near Baltimore, Maryland on March 13, 2019.
Jim Watson—AFP/Getty Images

Total boarding: 157,754,667
Passengers denied boarding: 6,411
Passengers denied boarding involuntarily, per million: 41

Southwest Airlines, meanwhile, bumped the most ticket purchasers by number. But it also boarded more passengers than any other airline — 158 million in a year, which is about 20 million more passengers than the next most boarded (Delta Air Lines). The small fraction puts it at third to last.

Alaska Airlines

People stand in the Alaska Airlines ticket area at Sea-Tac International Airport, in SeaTac, Wash., August 10, 2018.
Elaine Thompson—AP/REX/Shutterstock

Total boarding: 25,170,875
Passengers denied boarding: 703
Passengers denied boarding involuntarily, per million: 28

Alaska turned away the least amount of passengers of the five by raw number. But due to the relatively low number of passengers boarded, the 703 Alaska passengers who were denied boarding were 28 in a million — more than American Airlines or the three other airlines who didn’t even make this list.

American Airlines

An American Airlines aricraft sits at a gate at O'Hare International Airport on May 11, 2018 in Chicago, Illinois.
Scott Olson—Getty Images

Total boarding: 131,762,176
Passengers denied boarding: 3,115
Passengers denied boarding involuntarily, per million: 24

Similar to Southwest, American Airlines denied boarding to thousands of passengers, but the airline also boarded a lot of passengers. The more passengers being served, the higher the likelihood that one would get turned away — or so one would think.

Compare their stats to Delta, which served two million more passengers than American and only turned away three passengers for every million boarded. The other two airlines surveyed were United (13 passengers bumped) and JetBlue (just two passengers bumped).