Passengers were forced to disembark from a plane and wait alongside it while it was searched after a traveller boarded the wrong jet by mistake.
The man had been due to travel from Liverpool to Belfast with EasyJet at 07:40 GMT but instead boarded the 07:40 flight to Jersey.
The security error, which one passenger called "pathetic", meant the Jersey flight was delayed by about 25 minutes.
EasyJet said it was investigating how the mistake had happened.
After being taken off the Jersey flight, the passenger was allowed to board the plane to Belfast, which left Liverpool John Lennon Airport at 07:50.
A fellow traveller, who identified himself as TheFatTrader on Twitter, said everyone on the Jersey flight had to get off while staff checked that the man had not left anything behind.
He tweeted that they had been told there was "a security breach".
"A passenger boarded the wrong plane. Pathetic security," he added.
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An EasyJet spokesman said the firm "works closely with all of our airports to ensure the safety and security of all passengers".
"As soon as the mistake was realised onboard before departure, it was arranged for the passenger to board their correct flight.
"We are currently investigating how the passenger was able to board the wrong flight."
A spokeswoman for the airport said it was "working with the airline and handling agent to further investigate the circumstances".