The unexplained error had the giant airline selling plane tickets for about the same price as a trip on a Chinatown bus until United pulled the plug, The Associated Press reported.

Forbes.com saida user on an online forum called Flyertalk postedthe word of the ultra-cheap fares Thursday. (“Just went to book this for a work trip and ended up paying $7.50 all in,” said the posting, by someone who said he had just bought tickets from Dulles to Minneapolis-St. Paul.)

“Attention grew rapidly, with over 100 replies in just an hour, and the news spread to Twitter,” Forbes wrote, reporting that the fares were as low as $0 plus $5 in tax. “Some forum readers reported finding $10 flights between Washington DC and Hawaii, while others scooped up over a dozen tickets to destinations all over the country.

“And then, just as quickly as the airfares showed up, United’s reservation system slammed to a halt, reporting ‘United.com is currently undergoing maintenance,’” Forbes added.

United later said it had accidentally filed the fares for $0, and that it stopped taking bookings through its website and phone centers to prevent more of the tickets from being sold or given away, the AP reported. The site was accepting reservations again around 3:45 p.m. Eastern time, the AP said.

The news service quoted United spokeswoman Megan McCarthy as saying the mistake stemmed from an error in filing the fares, not a problem with the website.

The airline didn’t know how many absurdly cheap tickets it had sold — or whether it would honor them.

United didn’t immediately return a request from POLITICO for comment.

In the past, airlines have accidentally created super-low fares by dropping a digit when entering fares into a computer system, the AP said.