The fares advertised by Southwest Airlines sounded too good to be true: $59 for flights from Atlanta to Los Angeles, Chicago or New York.
When would-be fliers tried to book the fares, they found no seats available on those routes.
The U.S. Department of Transportation announced $200,000 in fines Thursday against the Dallas carrier for violating the federal full-fare advertising rule.
The regulation requires airlines to display the full cost of all fares, including taxes and fees. Airlines that advertise fares for specific routes must also offer a "reasonable numbers of seats" for those flights.
In addition to the fine issued Thursday, the agency ordered the airline to pay a $100,000 penalty that had been suspended after a previous violation of the same rule.
Southwest Airlines said the latest violation was the result of an error in a television ad. The airline said it never intended to offer the $59 fare for flights to New York, Los Angeles and Chicago.
"As soon as we became aware of our mistake, we pulled all incorrect advertisements off the air," the airline said in a statement.
In the previous violation, Southwest advertised $66 one-way fares from Dallas Love Field to Branson, Mo., last year from March 1 to March 21. However, the Transportation Department found that there were no seats available at the advertised price during the sale period.
The agency fined Southwest $200,000 for that violation but suspended half the fine on the condition that the airline "cease and desist" from violating the same rule for one year.