This was one of the most horrifying incidents in aviation history. In August 2005, Helio Airways Flight 522 was on a short trip from Cyprus to Greece, when it veered slightly off course. Nineteen radio requests were sent for an explanation but there was no response.
Finally, two F-16s caught up with the plane to see what was going on, and their pilots saw that the captain's chair was empty and the co-pilot appeared to be unconscious, with his oxygen mask dangling from the ceiling. All the passengers were frozen to their seats, at temperatures of minus 58 F (minus 50 C).
The fighters followed the "ghost plane" as it flew on autopilot until it ran out of fuel and crashed into a hillside. The official report by Greek authorities concluded that the pilots had somehow incompetently left the plane's pressurization system in manual mode after takeoff, so the plane didn't automatically repressurize when cabin pressure was lost. Further, the pilots failed to recognize the warning signs that the cabin was losing pressure -- and oxygen -- until it was too late. But that explanation fails to satisfy those who suspect that the aircraft was haunted [sources: Daily Mail, Krisch].