My dad was very kindly installing lots of door knobs throughout our house (bedrooms, closets, bathrooms…). He had done lots of doors and was in the final stretch when he was installing the privacy lever in my office door. After he had been working on it for about 10 minutes, I asked him whether he was placing the door lock on the inside of the room (as is correct) or the outside of the room (which would be backwards). He looked at me and laughed (since he had already made that mistake before) and kept working.
A few minutes later he realized that he was indeed installing it inside out.