United Airlines is cracking down on an up-and-coming ticketing trick. Have you heard of hidden-city ticketing? Passengers use this loophole to book trips to cities they don't actually intend to visit. The key is to book a cheap one-stop fare that features a stop in the place where you actually do want to go. An example would be booking a flight between New York and Los Angeles that stops in Denver and simply staying in Denver instead of completing the second leg of the journey. The tactic works because direct flights are notorious for being more expensive than one-stop flights.

United is making it very clear that hidden-city ticketing violates its contract of carriage. The airline wants customers to know that they must fly to the actual end-point destination that is attached to each fare that is purchased. How will United catch hidden-city travelers and stop them? The airline is calling on airports to help. United has supposedly asked agents working at airports to keep their eyes out for potential hidden-city travelers and report them to United's security department.

United has actually been battling the hidden-city loophole for years. United famously sued a website called Skiplagged that helps travelers find hidden-city itineraries that work for their travel plans. United has also contacted passengers directly to alert them of violations.

The carrier's corporate security department has supposedly sent letters to repeat-offense passengers asking them to pay back lost revenue after using the hidden-city loophole to save thousands of dollars. American Airlines has also supposedly sent similar letters to customers. It's not just airlines in the United States that are fighting back against hidden-city tactics. Lufthansa sued a passenger who skipped a leg of his trip earlier this year.

Scott Dylan is a contributing writer at GET.com and has been to (almost) every country in North, Central and South America with nothing more than a backpack, a laptop and the desire to explore. He speaks Spanish fluently and has logged enough time in planes, trains, rideshares, buses, taxis and rickshaws to know how to rack up rewards and points to get anywhere his heart desires for pennies on the dollar. Email: scott.dylan@get.com.