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For the second time in about a month, a malfunction on United Airlines' Web site is letting customers buy tickets on the cheap. Passengers can take advantage of the error by setting up a "MileagePlus" account and tricking the site into getting them a ticket with frequent-flyer miles they don't have, according to Mashable.

The news site said that it was able to reserve a roundtrip ticket from Newark, N.J., to Dublin, Ireland for $49.40, which covers the cost of taxes and fees. Apparently, the loophole is currently still open.

This isn't the first time United has experienced Web site errors to its detriment. In September, for about a one-hour period, the airline priced tons of tickets at $0. Some users reported booking multiple tickets, including Washington, D.C. to Honolulu and San Francisco to Newark, N.J., for nothing. One buyer told CNET that he was able to book a round-trip transcontinental flight for $0, which came out to $7.50 after applicable taxes and fees.

In the September glitch, the problem was reportedly because of a programming error. After United noticed the fire sale, it stopped all online booking for at least an hour. The airline said it would honor the tickets sold during that time. But users who were able to take advantage of Monday's malfunction aren't so lucky.

"We've identified an issue where customers are intentionally manipulating our Web site," a United spokesperson told Mashable. "We will not honor these reservations."

Similarly, in July of last year, United canceled a group of tickets after some Web site users were able to book tickets between the United States and Hong Kong for four frequent-flyer miles and up to $43 in fees. Normally, it takes at least 65,000 miles for a round-trip ticket to Hong Kong in economy class.

CNET contacted United Airlines for comment. We'll update this article when we get more information.