According to United spokeswoman Susana Leyva, the Chicago-based airline was running a special on flights from its hubs in Chicago and Denver. A further discount of $5 was to be applied to tickets purchased at the United.com site.
Instead of taking $5 off the fare, ATPCO posted the United sale fare as $5, plus tax. ATPCO is owned by a consortium of all the US airlines and serves as a clearing house for publishing air fares.
"We discovered the problem very quickly, within no more than 45 minutes," Leyva said. The United booking engine was shut down for 30 minutes after that until the problem was corrected.
It is not clear how many tickets were purchased during those 45 minutes, she said, but United will honour the $5 fare.
Leyva said she did not know whether the mistake was caused by human or computer error.
United had a similar problem in January 2001, when flights from San Francisco to Paris were offered for $24.98. Initially, the airline said it would not honour the fares, but later agreed to do so.