Best all-time screw-up: In a rush to head out the door, I had my nephew load a pair of A-12 backs for me. I showed him to load, thought he had it, and went on to start carting equipment out to the car. Later on in the day, I clipped on one of the backs he had loaded. First couple of "shots" - no problem. Then at frame number four, the crank refused to budge. So, I gave it a little help. A tearing sound, and then no resistance. Oh s..t! Knowing the roll was lost, I pulled the dark-slide and then opened the back. The film had been loaded reversed: rather than black side out, he had loaded with cover ("Kodak Ektachrome 100G") facing outwards. The only saving grace: by sheer luck of the draw, the two backs were the last of six I had taken on the shoot, so the net loss, after I wrecked a roll, was only one (I saved the second roll when I returned home). Lesson learned: explain carefully and be certain your student has it!