Too good to be true: New York to Hong Kong for $43

(CNN) -- A first-class return ticket from New York to Hong Kong for $43 and four air miles sounds too good to be true. And when Brian Kelly purchased such a fare -- which usually costs about $11,000 -- using United's online ticketing system last week, he knew that's exactly what it was.

But he also knew from first-hand experience that "error fares" -- rock-bottom fares mistakenly issued by carriers -- can sometimes be recognized as good for travel by airlines.

Having booked two fares at rock-bottom prices on the site, Kelly, who runs an advice website for frequent flyers called The Points Guy, alerted his readers to the "amazing deal" on his website, encouraging them to take advantage of the offer before United corrected the glitch -- then waited to see if the ticket would be honored.

"Everyone knew this was a mistake -- a huge one. One of the biggest I've seen an airline make," he said.

Airlines regularly make errors resulting in mispriced fares being issued, sometimes honoring them but often not. "I've been on a lot of crazy mistake fares in the past that did work out, the airlines did honor them," said Kelly, who recounts how he recently flew round trips from New York to Stockholm and Copenhagen on three consecutive weekends for $138 a trip.

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Kelly said he was optimistic that although the fares were clearly issued in error, United would be required to honor the tickets following a recent ruling on "mistake fares" by the U.S. Department of Transportation.

The rule means that a purchase occurs once the customer pays the full amount -- and that once the purchase is confirmed, whether through an email or credit card transaction or other means, "then the seller of air transportation cannot increase the price of that air transportation to that consumer, even when the fare is a 'mistake.'"

Kelly had received a confirmation email from the airline, suggesting the fare should be valid under the new rules. But United contacted him and others to tell them that the incorrect fares had been issued due to a "programming error." Customers who wished to retain the fares would be charged the full amount, or could cancel the purchase and have any fees waived.

United spokesman Rahsaan Johnson said it would have been clear that the fare was a mistake as the correct amount was displayed on the same screen. On website Flyertalk, United said the fact that the correct price was displayed was a "unique circumstance" unlike "other widely reported 'mistake fares.'"

However, for customers who had already begun their travel by the time the glitch was detected, the airline would honor their return fares. The airline would not say how many people booked flights for the incorrect price, or how many actually managed to travel on the cheap fares by commencing their travel before United took action.

Kelly said although he "would have liked the airline to have honored" the fare, he wasn't going to lose any sleep over missing out. But he said it would have provided a good test case for the Department of Transportation.

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Department of Transportation spokesman Bill Mosley said the department had received 150 complaints from customers who had bought the "error fares" and was still determining whether United had violated the rules. "If we find a carrier violates our rules, we can issue fines and cease-and-desist orders," he said.

Airline responses to pricing mistakes are unpredictable. In 2010, American Airlines refused to honor first-class return tickets from the U.S. to Australia, worth up to $20,000, it had sold for the coach price of $1,100. But it offered $200 vouchers as compensation. The previous year British Airways also failed to accept $40 flights to India, offering $300 vouchers instead. But in 2008 Swiss International honored some $0 flights booked between Toronto and destinations in Europe and India, while refusing others.

Kelly polled his site users and asked them if they thought taking advantage of such mistakes was unethical. While about a quarter thought those who booked the tickets were "bottom feeders trying to ruin a poor airline," he said, the majority was comfortable with opportunistically capitalizing on the mistakes of airlines.

What do you think? Would you knowingly book an "error fare" with an airline? Or is doing so unethical? Let us know in the comments section below.