Previously, I reported finding a hair-like pattern on the edge of a frame. I got lots of helpful suggestions, but I've finally found the source of the problem. It happened again, and this time I looked very carefully at the offending film holder. On the cloth hinge at the flap end, some very fine threads had come loose and were dangling. These were long enough so that they could intrude in the film area with the dark slide removed. The film holder was one of two Riteway holders, and the problem only occurred on one side of one of them. The cloth hinges for my Fidelity holders are glued only on the end, but those for my Riteway holders extend over the edges.
I've now removed the threads, and I will be careful to check my film holders in the future.
Does anyone know what the hinge material is?