After flight rather than pre flight. An S76B belonging to a company I used to fly for was being cleaned in the hangar after flying all day. As the aircraft cleaner worked on the tail pylon he thought he felt it move. On investigation it was discovered that the only thing holding the top two thirds of the entire pylon (and gearbox plus tail rotor assembly) in place was the right side skin of the main box section - all the rest of the structure had completely failed. A lucky escape for the pilot (who occasionally posts on this very forum).