When Air Canada implemented its new pricing structure, it also eliminated many travel restrictions that passengers hated, such as requiring a Saturday-night stay and a round-trip ticket to get the lowest fare.

CNN: Pros and cons of a la carte airfare pricing

Delta says it, like other airlines, offers all seats on planes to frequent fliers, just at higher mileage cost than the lowest redemption level, which is 25, 000 miles for a domestic coach round-trip ticket.

WSJ: Frequent-Flier Miles: Best Airlines for Cashing In

In addition, Mr. Abdulmutallab's cash purchase of a round-trip ticket didn't trigger any secondary screening, calling into question the efficacy of the system that is supposed to spot passengers who are acting suspiciously and may be threats.

WSJ: The Middle Seat | How to Improve Travel in 2010

Tengo bought a Tokyo-Chikura round-trip ticket.

NEWYORKER: Town of Cats

If your presidential candidate loses the election in November, and you win an online contest, JetBlue Airways will give you a free round-trip ticket, one of 1, 006, to escape to Mexico or Caribbean or other international destinations and ease your pain.

FORBES: JetBlue Offers Free Flights For Voters Who Want To Skip The Country If Candidate Loses

Umar Farouk AbdulMutallab, the man accused of trying to blow up a U.S. airliner bound for Detroit, Michigan, had a one-way ticket from Accra, Ghana, to Lagos, Nigeria, and a round-trip ticket for his trip from Lagos to the Netherlands and then to Detroit, according to the Nigerian Aviation Authority.

CNN: Are dogs the key to bomb detection at airports?

It made little sense to him to rent a car or buy a round-trip Amtrak ticket to get to Washington instead of simply flying where you want to go.

WSJ: Decoding Airline Ticket Costs

Look for somebody who's bought a one-way ticket, or look for somebody who's bought a round trip ticket.

NPR: Airport Security Marked by Hassles

One, completing a round trip, and the other, it's off on a one-way ticket.

NPR: Stardust Set to Deliver Comet Samples