Boarding passes are full of data about the passenger who is holding them, not just about where they plan on travelling to.

While most codes printed on your boarding pass are harmless, there is one that may indicate bad news for you as a traveller.

According to an Air Canada ticket agent, there is one code you don't want to have on your boarding ticket - "GTE".

If you have GTE on your ticket it reveals your flight has been oversold.

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"If someone has GTE (for 'gate') on their boarding pass, it means they don't have a seat," the ticket agent worker told CBC.

"I train people to dupe passengers."

He explained airlines maximise revenue by selling more plane tickets for a flight than there are available seats.

The unnamed attendant also claimed he told airline ticket agents not to tell passengers as they would get upset — instead, they should send them to the gate.

The revelation came after Air Canada employees revealed how they were trained to hide information from passengers regarding their flight ticket and whether they had been given a seat.

The former Air Canada agent said he's gone public because he wanted travellers to know how often staff are forced to scramble to find seats for passengers stuck on oversold flights.

He explained he soon left the industry saying he couldn't continue lying to customers.

"I told them they had nothing to be worried about, and it absolutely killed me," he said. "The chances of them making it on [the flight] were slim to none."

He revealed some of the stories of travellers being "bumped" from flights, such as a couple on a honeymoon together or a family on their first holiday.

In an email to CBC, Air Canada spokeswoman Angela Mah disagreed with the allegations and played down the impact of overselling.

"Overselling ... accounts for less than 1 per cent of passengers booked," wrote Mah, explaining that the airline flew approximately 51 million customers in 2017/18.