Carrying two small bags, Spears, 25, ran through Miami International Airport to catch her American Airlines flight. But when she arrived at the gate and handed her boarding pass to the ticket agent, she was informed that the walkway had been retracted from the plane and the flight was closed.
"She was really upset," says David Paulsen, a passenger at an adjoining gate. "She (said) to the agent, 'I've got to get on this plane to get home to my kids.' She looked like she was about to cry."
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When Paulson later approached the ticket agent about Spears, he was told that it was, indeed, the pop star and she had gotten one of the very last seats on a full plane, with first class entirely booked.
An American Airlines spokesperson tells PEOPLE that it's not unusual for the airline to allow late arrivals on the plane as long as it doesn't delay the flight. The final decision is left up to the cockpit crew and gate agents.
As a visibly relieved Spears was about to board the plane, Paulson says, she stopped to give an autograph to the agent, and then turned to the crowd of onlookers and waved. "'God bless!'" she said, as she got on the plane.