It was the New Year’s Eve travel sale that appeared to be too good to be true: business and first-class flights on Cathay Pacific from Vietnam to New York for $675 (£534), rather than the standard $16,000.

It turned out to be a ticketing error, but the airline promised to honour the sale. “Happy 2019 all, and to those who bought our good – VERY good surprise ‘special’ on New Year’s Day,” the Hong Kong-based carrier tweeted on Wednesday. “Yes – we made a mistake, but we look forward to welcoming you onboard with your ticket issued. Hope this will make your 2019 ‘special’ too!”

Cathay Pacific (@cathaypacific) Happy 2019 all, and to those who bought our good - VERY good surprise ‘special’ on New Year’s Day, yes - we made a mistake but we look forward to welcoming you on board with your ticket issued. Hope this will make your 2019 ‘special’ too!

.#promisemadepromisekept #lessonlearnt

The flight offer was picked up on New Year’s Eve by Gary Leff, a travel blogger, who wrote: “Oh my goodness this is an amazing fare.

“You can fly from Vietnam – the cheapest fares originate in Da Nang, but other cities work as well – to Cathay Pacific’s US gateways starting at $675.”

Leff warned his readers not to book any additional travel or hotels around the deal, as he suspected Cathay “might get seller’s remorse” and not honour the fares. “I’d wait a few days after buying additional travel or making additional non-refundable plans around the fare,” he said.

But when Cathay became aware of the mistake, the airline said it would stand by the fares. “We do not want to go back on our promise to our customers,” it said. “We made a mistake but we look forward to welcoming you onboard with your ticket issued.”

The airline did not respond to requests for comment about the number of flights sold at the steep discount. Cathay has removed all business and first-class flights for August – the month of the cut-price fares – from its website, but the same flights in September are listed for $16,000.

Another blog, One Mile at a Time, saw it was possible to buy first-class flights from Hanoi to Vancouver for $988 return.

In a blogpost, it said: “Wow! Hurry, this won’t last.” There were also first-class return flights from Hong Kong to New York offered for $1,450. The price of those flights in September is more than $31,000.

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The South China Morning Post tracked down 11 passengers who collectively bought 18 first and business-class tickets at the discounted prices. They paid a total of £21,700 for flights that should have cost £540,000.

It is not the first time airlines have mistakenly offered premium-class flights at economy prices. Singapore Airlines made the error in 2014, as did Hong Kong Airlines last summer, and both honoured the errors. But in 2015, United Airlines cancelled hundreds of tickets it sold for $100 by mistake. United said it would not honour the fares because the error was caused by a “third-party software provider”.

Earlier this year, Cathay had to repaint one of its jets after customers noticed the airline had spelled its name wrong. The Boeing 777-367 was emblazoned with the words “Cathay Paciic”.