How exactly does a passenger get on the wrong flight?

How exactly does a passenger get on the wrong flight?  CNN Chrissy Teigen tweet-storms about surviving her 8-hour flight to nowhere, airline apologizes  USA TODAY A passenger boards the wrong plane. So Chrissy Teigen tweets about an 8-hour 'flight to nowhere.'  Washington Post Passenger Aboard 'Incorrect Flight' Forces Plane To Return To LAX  CBS Los Angeles Tokyo-bound flight with Chrissy Teigen forced to return to Los Angeles  NBCNews.comFull coverage