When the UK regulations were changed in the 1960s to allow private owner's to do more simple maintenance tasks on their aircraft, a 3-day course was run at Cranfield for PPL holders, to teach them how to do it properly.
At the end of the course, all students were invited to pre-flight a single-engine 4-seater, fixed tricycle undercarriage, fixed pitch propeller, I forget what type it was, which had been "prepared" for this exercise. Most of us got most of the visible defects that had been set up, but not one of the 20+ new experts noticed the fact that the propeller was installed back to front, even though we all spent about 20 minutes on the inspection.