Tickets that are purchased in an order to violate minimum stays or other requirements are classic examples. American Airlines offers easy definitions in their examples if prohibited practices in its Conditions of Carriage..

Back-to-back ticketing: The combination of two or more roundtrip excursion fares end to end for the purpose of circumventing minimum stay requirements.

Throwaway ticketing: The usage of roundtrip excursion fare for one-way travel.

Hidden city / point beyond ticketing: Purchase of a fare from a point before the passenger's actual origin or to a point beyond the passenger's actual destination.

Duplicate and impossible/illogical bookings: Duplicate or impossible/illogical American Airlines bookings are prohibited without prior authorization from American Airlines. A duplicate or impossible/illogical booking includes, but is not limited to, bookings for the same passenger on flights traveling on or about the same date between one or more of the same or nearby origin and/or destination (such as JFK to DFW and LGA to DFW or DFW to LAX and DFW to ONT), or bookings with connections that depart before the arrival of the inbound flight.

Fraudulent, fictitious and abusive bookings: Fraudulent, fictitious and/or abusive bookings are prohibited. These types of bookings are defined as any bookings made without having been requested by or on behalf of the named passenger. Additionally, creating bookings to hold or block seats for the purpose of obtaining lower fares, AAdvantage award inventory, or upgrades that may not otherwise be available, or to gain access to airport facilities, or to circumvent any of American Airlines' fare rules or policies, is prohibited without prior authorization from American Airlines.

(An example if the last example might be making a booking to enter an airside lounge, and cancelling after entering.)