United Airlines was selling some tickets on Thursday for as little as $5 before shutting down bookings on its website.
Some customers were able to buy tickets before the airline stopped accepting orders on the website. A message on Thursday afternoon said flight searches and bookings were shut down for all flights. United also is not taking reservations by phone.
A United spokeswoman didn't have an immediate comment.
Airline fare mistakes have happened before, often when an airline drops a digit when entering fares into its computer system. Social media such as Twitter ensures that word of such mistake fares spreads even faster than before.