Many years ago, I was shooting a wedding in a large hotel with a Mamiya-Sekor twin-lens-reflex camera.
During a break in the activities, I met a colleague who was shooting another wedding in an adjoining ballroom. He commented on my camera and asked if he could see it because he was thinking of buying one. I carefully watched him as he inspected my camera to make sure that he did not accidentally drop the camera or open the back and expose the film.
After we parted to return to our respective weddings, I quickly checked my camera to make sure he had not changed by exposure settings. My settings were correct; however, I noticed that he had changed my sync setting from electronic flash (X) to flash bulb (M). Had I not found this change, the rest of my flash shots would have been ruined.