( 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 . See also : 'Hidden ' airline charges just dirty tricks ? 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 can not 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 was n'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 . See also : Fliers pay more for window seats . 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 .
A bug in United 's ticketing system made for cheap air fares last week . United said it would not collect the fares , except for passengers who had already departed . New regulations say airlines must honor tickets issued in advance , once paid for .