Bart Jansen, USA TODAY
Even low-cost carriers can't compete against this: United Airlines on Thursday was briefly giving away tickets for free or negligible fees.
United says it accidentally filed fares for zero dollars, although airport charges could have resulted in a small cost. Passengers reported buying tickets for $5 to $10.
The airline temporarily stopped taking bookings through its website and phone centers to prevent more sales. But the website resumed accepting reservations about 2:45 p.m. Central time.
"One of our filings today contained an error which resulted in certain fares displaying as zero," said United spokeswoman Megan McCarthy. "We have corrected this error."
The Associated Press contributed to this story.