The hijacking of commercial aircraft, as part of the September 11, 2001 attacks, has made nations more diligent about keeping track of what's flying into their airspace. Another example of that heightened vigilance occurred recently as an Air France airliner flying from Paris, was entering Indian air space on its way to Thailand. The Air France crew had typed in the wrong ID code for their transponder.
Like military aircraft IFF (Identify, Friend of Foe) transponders (which are encrypted), the civilian version transmits an ID to traffic control radars, so controllers know who's up there. The Indian controllers noted that Air France flight they were expecting, was instead sending out a different code.
So an Indian Air Force MiG-29 was sent up to visually check out this mystery aircraft (and shoot it down if it turned out to be controlled by terrorists and heading for an Indian target.). But just as the MiG was about to reach the Air France jet, air controllers contacted the aircraft and sorted out the transponder code mix up. The MiG was recalled and all was well. This time.