LONDON — For travel bargain hunters, the first day of 2019 was a spectacular one.

On routes between Asia and North America, Cathay Pacific, the Hong Kong-based airline, offered seats in its exclusive business and first classes for less than the economy fare. A round-trip flight in business class from Vietnam to New York cost as little as $674.63, a small fraction of the usual price, according to One Mile at a Time, a travel blog that documented the fares.

“Cathay Pacific does actually do some quite aggressive fare sales from Asia,” said Rob Burgess, the editor of a frequent flier website, headforpoints.com. Business class seats from Asia to the United States can be found for around $5,000.

But not this aggressive.

The low prices, offered by mistake for several hours on Monday in North America, caught Cathay Pacific itself by surprise.

On Wednesday, the airline acknowledged the error but said it would honor the fares. On Thursday, the trip that had briefly cost less than $700 appeared on the Cathay Pacific website for about $16,000 for a fully flexible fare.