A botched jump in which a man fell 1,500ft without a parachute was dismissed by British air traffic controllers as a practical joke.
The pilot had asked controllers at Coventry Airport for permission to "throw a passenger out" before David Clements made his jump.
The 45-year-old hit his arm on the way out and could not open his parachute. He survived after landing on the roof of an aircraft hangar.
A report by the Air Accident Investigations Branch said air traffic controllers thought the "small bundle" they had seen falling from the plane was a joke.
They realised what had happened 20 minutes later when fire and rescue crews arrived at the scene.
Firefighters spent 90 minutes freeing the mechanic from Dunkeswell, Devon, from the plastic roof. He suffered internal injuries and a broken arm and ribs.
The report said: "Air traffic controllers had received no prior notification of a parachute drop and because of the informal nature of the request, initially regarded it as a joke."