I have discovered what caused the streak extending through the middle of the negatives of one of the participants from my recent workshop.
It was my fault. Once again, it was the result of an old habitual behavior just expressing itself without me realizing it.
To understand what happened, I had given John another roll of film and he agreed to expose it and send the camera to me.
I always load film in the right bay so the numbers are upright. I know I told them it didn't really matter which side it was loaded on but repeatedly mentioned it, demonstrated loading that way and showed them the dots going by with right side up numbers. I just expected them to load it that way. John had loaded the film in the left bay, the way he's accustomed to doing it in his 35mm Minolta.
When I received the camera, I could see backing paper was still visible in the counter window. Before unloading the camera, I thought I was winding it all the way onto the take-up spool with the left winder, but I was rewinding. This will cause the film, unnattached to the paper at the end of the last frame, to catch on the internal divider and spool into the image chamber. About when I thought I had wound it too long, I saw shiny film in the counter window! I immediately closed it and went into the darkroom. With the camera open, it all just felt bunched up. I really couldn't tell how it looked.
I must have done exactly the same thing at the museum, although I can't visualize exactly what's happening in the camera. I don't remember any of this when I wound the film on the reel in the class, but we had the other problem with the changing bag.
The streak appeared on the last two frames. Much to my relief, ten frames survived.
I've tried to replicate this by reattaching and rerolling a dummy roll of film to some backing paper. Every time the rewinding just bunched it up inside the image chamber until it jammed and nearly forced the camera open with the backing paper still visible in the window. I don't understand how that film got exposed to the counter hole but I'm not going to sacrifice a roll of FP4+ just to find out. I know what the cause was and how to avoid the problem.
I was happy John would get to see the photographs he took. Here are a few examples.
Some flowers still looking interesting long past their prime.