Remember the DC-4 (Air Pennsylvania, I believe) in the 1940's???
Actually, I think it was American Airlines near El Paso, here's a contemporary account from Time magazine:
TRANSPORT: Boys Will Be Boys
Monday, Oct. 27, 1947
Fortnight ago the heads of American Airlines faced an embarrassing task—explaining why one of their DC-4's had gone into a violent dive, on a clear, calm day near El Paso, had flown upside down, and dumped 48 fear-stricken passengers* out of their seats. After some consideration they decided not to talk at all. But last week the Civil Aeronautics Board revealed the simple, if startling, truth. The whole thing had been a, witless practical joke.
Its perpetrator was the plane's veteran chief pilot, Captain Charles R. Sisto, of Los Angeles. Captain Sisto was riding as a check pilot while another pilot, Captain John Beck, familiarized himself with the route. As the plane snored west at 8,000 feet, Sisto reached down from a jump seat behind Beck and fastened the gust lock—a device used to lock the rudder, elevator and ailerons while the plane is on the ground.
The plane began a steady climb. Puzzled, Pilot Beck adjusted trim tabs on the plane's control surfaces to bring the nose down. Then, still undetected, Sisto released the gust lock. The plane immediately went into an outside loop. Both Sisto and Beck, neither of whom had fastened his safety belt, were thrown from their seats. Two things saved the plane. Sisto struck buttons which feathered the propellors of three engines. Copilot Melvin Logan, who was securely belted in, was able to roll the ship right side up, a bare 300 to 400 feet from the ground.
Captain Sisto resigned (many airmen thought he should have been fired, many others thought he should have been jailed). If the plane had crashed, killing passengers and crew, it would doubtless have been added to the list of unexplainable accidents.
*A Frenchman, doused with the contents of the plane's chemical toilet, was apologetically informed: "This is not normal operating procedure in American airplanes."
TRANSPORT: Boys Will Be Boys - TIME
Sisto's pilot licenses were pulled and he appealed unsuccessfully:
179 F2d 47 Sisto v. Civil Aeronautics Board | Open Jurist
Apparently, he later got at least a commercial license back since he flew as a copilot for Transocean Airlines a few years afterward (another version of the gustlock incident is here):
One of the More Unusual Pilots a