A British man flew from the Czech Republic to Newcastle on the wrong passport after accidentally taking his friend's and leaving him stranded.
Allan Poole, 43, from Whitley Bay, travelled from Prague to Newcastle via Amsterdam on a KLM flight.
The passport was checked at least four times during the journey, but the mistake was not picked up.
KLM described the incident as "undesirable", but stressed no passengers or crew were ever at risk.
Mr Poole's friend, Steve Vincent, 43, also from Whitley Bay, is now getting help from UK officials to get home.
It has emerged that Border control agents who checked Mr Poole's passport when leaving the Schengen area at Schiphol airport, Amsterdam, and UK immigration officials who checked it when he arrived in Newcastle did not identify the mistake.
KLM staff at the boarding gate in Amsterdam also did not notice the name on his boarding card did not match the passport.
Mr Vincent tweeted KLM asking, "Hey KLM you let my friend travel from Prague to Newcastle on my passport (after we swapped by mistake).
"You shouldn't have let him on the plane. I'm trapped in Prague. What are you going to do about it?"
In a statement, the airline described the incident as "undesirable".
It added: "One of the passengers flew from Prague to Newcastle via Amsterdam, without it being noticed at the various airports that he was travelling on someone else's passport.
"KLM works with competent authorities to ensure that the aircraft, passengers and crew are safe and secure during flight.
"Although this is an undesirable, but exceptional situation, the passenger went through all security checks. The safety of passengers and crew has never been compromised."
A spokesman for the Royal Netherlands Marechaussee, which is responsible for border checks at Schiphol Airport, said: "When mistakes do happen, we are committed to acknowledging them and putting things right for the future."
A spokesman for the British Embassy in Prague said it was not unusual for people to travel on other people's passports.
He said: "People do travel on (the wrong passport), they don't get checked when they come out and stuff like that.
"It's quite common actually, to be honest with you, people do travel on other people's passports accidentally, it does happen.
"Border controls sometimes don't look, I don't know for what reason."
Mr Poole said no-one noticed he was travelling on the wrong passport and he only realised himself when he got back to the UK.
The British Embassy spokesman said staff would assist Mr Vincent in obtaining an emergency travel document.