A United flight to Beijing was diverted after they found out there was a parent on board traveling out of the country with her child with possibly no intention on returning back to this country:
The United Airlines flight departed Virginia’s Dulles International Airport at 12:39 p.m. When it reached Canadian air space, the FBI ordered the Boeing 777 back to Virginia after learning a kidnapping suspect was on board.
Flight 897 landed back at Dulles five hours after its departure.
The interesting part to me was that the pilot told the passengers that there was a mechanical error to keep things calm and to keep the suspect completely unaware.
According to the article, if the plane diverted into Canada instead of turning back to the United States, this would have been a much more complicated case.