NEW YORK (AP) — It was an airfare deal too good to be true: fly first class to Hong Kong for just 4 frequent flier miles and $33 in taxes.

Clearly, it was a computer glitch. But it's also turning out to be the first major test of the Department of Transportation's new consumer protection rules prohibiting airlines from "increasing the price after the consumer completes the purchase."

On Sunday, computers at United Airlines erroneously let passengers book flights to Hong Kong — or other places in Asia connecting in Hong Kong — in exchange for 4 miles, plus government taxes. Frequent fliers quickly shared the error on blogs and online chat rooms. Some, such as Ben Schlappig who runs the site One Mile at a Time, advised people not to call the airline, saying, "there's no need to bring further attention to this pricing."

A business class seat for a flight on United to Hong Kong goes for about $8,500 or 120,000 frequent flier miles; first class costs $10,250 or 140,000 miles.

Before long, hundreds, if not thousands, of fliers — the airline won't say how many — booked trips. United eventually pulled the plug and announced it wasn't honoring tickets already sold. People could get a refund without paying a penalty or have the proper amount of miles deducted. Anyone who had already started their trip would be allowed to complete their travel.

Several people who booked tickets complained to the DOT, which is now investigating.

"Our rule on post-purchase price increases applies to frequent flier tickets, particularly when they also entail cash payments," DOT spokesman Bill Mosley said via email.

There's one sticking point in this case: the cost advertised was actually correct. A ticket searcher initially saw a cost of 120,000 miles. It was only when customers went to book that the 4 mile figure appeared. And if customers had the full 120,000 miles in their accounts, that was actually deducted. Those with less had no miles deducted. All passengers were charged the appropriate taxes.

Mosley said the agency hadn't yet "reached any conclusions."

The maximum penalty per violation is $27,500 but the government has wide discretion in what amount to actual fine. DOT says each ticket sold by United could be considered a separate violation.

Susan Clarke, a 62-year-old retired teacher from Aiken, S.C., is one of the people who bought a ticket. She knew it was a computer glitch but booked a trip for March anyway.

"United just made a big mistake and needs to honor it," she said. "That was their mistake, wasn't it?"

But those who make a living off finding airfare sales say there is a difference between a good deal and taking advantage of a mistake.

"When a waiter adds up the check wrong in my favor, I let him or her know. When a clerk hands me back too much change, I give it back," said George Hobica, founder of AirfareWatchdog. "These fliers knew that this was a mistake, and they should treat an airline the same way they treat any other entity."

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Scott Mayerowitz can be reached at http://twitter.com/GlobeTrotScott.