There is a famous story fatal accident involving a 206 and rotorhead MX. About 15 yrs ago in Canada mechanics were working on something with the head and had the "Jesus Nut" off the top. It was replaced, but then later removed by a different mechanic to put some primer or paint on it. The pilot for the a/c had witness or talked to the mechanic the day before that had reinstalled it. Long story short, the next day he takes off without the "jesus nut". The aircraft hovered and flew for over 10 minutes, then the head came off. There was enough grip between the splines and head from the torque, but once he reduced power or got in a low enough CG to break the tension, that was it.
But moral to the story, do a preflight. The pilot obviously didn't do a full preflight. He may have done a walk around, but he didn't get up on a/c and look at the head. Also, the mechanic that removed the nut never wrote it up or told anyone--he was not present the next day to stop them from flying.
Sounds like this is the same deal. I have witness people take off with panels open, gas caps off, seat belts hanging out, ground handling wheels attached, oil dipsticks removed, etc. Always do a preflight, and always do a final walk around before getting in. And have a system to remove an aircraft from service (placard, squawk sheet, whatever); then a system to return an aircraft to service involving the mechanic AND pilot.