For a few hours on the first day of 2019, the Hong Kong-based airline Cathay Pacific drastically — and unintentionally — slashed prices for first- and business-class seats on certain flights. A ticket from New York City to Da Nang, Vietnam, for example, suddenly cost $675, down from its usual price of $16,000.

The glitch was first spotted by Gary Leff, a travel expert who runs the blog View From the Wing. “Since we don’t know for certain how Cathay will feel about such a deep discount airfare sale, whether they might get seller’s remorse, I’d wait a few days after buying additional travel or making additional non-refundable plans around the fare,” Leff wrote.

As it turns out, the massive discount wasn’t the result of Cathay feeling the holiday spirit, but a glitch. And as surprising as the price cut was, the fact that this happens relatively often may be even more so.

The Cathay discount wasn’t the first time an airline accidentally slashed prices

The Cathay glitch was newsworthy because of the steep discount — more than 95 percent on some flights — and the amenities offered on board, which include in-flight champagne and caviar. The South China Morning Post tracked down 11 people who purchased 18 first-class and business-class tickets, which normally would have cost a total of $5.4 million Hong Kong dollars, or $685,800 US, for just over $216,925 Hong Kong dollars, or $27,000 US.

It’s unclear how many other people purchased tickets during the error, but the Post reported that “several thousand tickets” are believed to have been sold. Leff was right that Cathay could have refused to honor the discount, particularly given the amount of money it was set to lose on these flights. Instead, the airline tweeted that it would honor the discount.

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 — Cathay Pacific (@cathaypacific) January 2, 2019

This isn’t the first time an error like this has happened — or that an airline has agreed to honor heavily discounted first-class fares. Hong Kong Airlines made a similar mistake in 2018, mistakenly selling $600 round-trip tickets. Singapore Airlines sold 400 business class seats at economy-class prices in 2014. In both cases, the airlines agreed to honor the cheap fares.

But airlines don’t always let customers keep unusually affordable tickets. In 2015, United Airlines canceled thousands of bookings after a glitch let customers buy first-class tickets for as little as $100. The airline said the error was caused by a “third party software provider” and refused to honor the tickets.

Steep flight discounts are often the result of technical glitches — or, in some cases, human error

According to the South China Morning Post, the Cathay discount fares were the result of a person entering the wrong fares into the company’s system for flights between Vietnam and North America. The 2015 United glitch was similarly the result of a software vendor who put in the wrong exchange rate when converting Danish kroner to British pounds, causing $4,000 first-class seats to sell for less than $100.

Tarik Allag, the founder of the website Secret Flying, explained that these kinds of errors are relatively common and have three root causes. One is human error, like the Cathay and United Discounts. In other cases, heavily discounted flights are the result of missing fuel surcharges on tickets.

Plane tickets have several components: One is the base fare, or the cost of the flight itself. Taxes are another component, and those vary from place to place — US flights charge a “Passenger Fee,” also known as the September 11 Security Fee, for example. Then there’s the fuel surcharge, which, according to Allag, was introduced in the ’70s in response to rising oil prices.

“Economy class error fares are usually the result of a fuel surcharge self dump,” Allag told me in an email. “This is because the base fare tickets on economy flights can be extremely low (sometimes as low as $1). So if the fuel surcharge is missing, the overall price of the ticket can be ridiculously low.”

You’ll never find a first-class or business-class ticket for $1, though, because the base fares for those seats are higher, Allag explained. “The cheapest business class error fares are when the airline mistakenly enters the incorrect base fare. For example, an airline can accidentally forget to add the extra 0 and set the base fare at only $150 instead of $1500,” he said.

Fare errors are also occasionally the result of online travel agency (OTA) glitches. “Not all OTAs will have the same error fare, so the more OTAs you go through, the better the chance of finding an error fare,” Allag said. But, he added, there’s “no real strategy” to finding these fares — they happen sporadically and tend to be spotted quickly. Act fast and you could get lucky. Unless the airline refuses to honor the fare, that is.