The cheat is known as "skiplagging", with a passenger booking a flight from A to C via B, but with the intention of never making their B to C flight.
Instead, the unnamed passenger got off during a layover in Frankfurt and flew on a separate ticket, also with Lufthansa, to Berlin.
Leading airlines are fighting back against passengers who use a simple loophole to secure cheaper tickets.
Germany's Lufthansa Group Airlines is suing a passenger who found a cheap way to travel between several cities in Europe and the US, saying the customer broke its rules when he skipped part of his return flight on a round-trip ticket from Oslo to Seattle.
The German airline Lufthansa sued a passenger in 2018 after he failed to board the final leg of his flight from Seattle to Oslo, CNN reports.
For example, someone flying from New York City to San Francisco could book a cheaper flight from New York City to Lake Tahoe with a layover in San Francisco - then simply get off there.
Lufthansa are hit harder by the practice than other airlines because many of their flights are routed through the hubs of Frankfurt and Munich.
The hack took advantage of the airlines' ticketing system, whereby multi-stop journeys are typically cheaper than non-stop flights. It is now seeking $2,385 in compensation.
In the Lufthansa case, the airline sued the man and sought 2,112 euros ($3,370) compensation for the violation of its terms.
A Berlin district court reportedly dismissed the lawsuit in December, but Lufthansa told CNN it has appealed the decision. But Orbitz later settled its case with the site, and a judge dismissed United's portion of the suit in 2015 - mostly because the case was filed in IL and Skiplagged is based in NY.
The principle that airlines charge less for more flights, lies behind pricing strategies by "network carriers" such as Lufthansa, Air France and British Airways.