The Department of Transportation is investigating United Airlines after the carrier sold tickets Sunday to and from Hong Kong for only four frequent-flier miles plus taxes, then announced it wouldn’t honor those reservations. It may be the first major test of a new pro-consumer, ticket-buying regulation.

In January, the DOT prohibited airlines from increasing the price of air transportation after a purchase was made, and also included airline “mistake fares”: once a confirmation is issued, “the seller of the air transportation cannot increase the price of that air transportation to that consumer, even when the fare is a “mistake.”