An airline had to cancel flights for thousands of passengers after it cleaned some planes' seat covers with water

Thousands of people had their flights canceled after an airline found it may have broken fire safety regulations by cleaning plane seats with water.

Finnair canceled around 40 flights on Monday and Tuesday, affecting some 5,000 passengers, an airline spokesperson said in a statement to Business Insider.

The Finnish flag carrier temporarily suspended operations for eight of its 15 Airbus A321 jets, they added.

This was because Finnair received information from the manufacturer of the planes' seat covers, saying that it hadn't been "properly verified" whether washing them with water affected fire protection.

"Safety is always our top priority, and we always follow the manufacturers' maintenance instructions as well as the guidelines and recommendations of the authorities," the spokesperson said.

On Monday, the eight jets were flown without passengers to Finnair's hub in the capital, Helsinki, they added.

"We are investigating options so that we can return the aircraft to service as soon as possible."

This isn't the first time fliers have had their plans disrupted after an airline became aware of a rare issue.

In June, a KLM flight U-turned over the Atlantic Ocean and returned to Amsterdam, after the airline realized the plane would reach its deadline for maintenance.

"To prevent the license from expiring during the flight, it was decided to return to perform the maintenance in the Netherlands," a KLM spokesperson previously told Business Insider.

Also that month, an American Airlines flight bound for Naples, Italy, changed course to Rome after the airline appeared to send a plane that was too big for its intended destination.

The airline used a Boeing 787-9 instead of its usual 787-8 on that route. The latter is about 20 feet longer and, therefore, has different requirements for rescue and firefighting services.

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