I had a couple of rolls sitting on my shelf for some time, and when I finally processed them several frames had these lines across the picture. At one point I blamed my camera, but then the effect would be consistent across multiple rolls, which was not the case.
And then I heard guys on the FPP podcast talking about light piping. Turned out it is when an unexposed or exposed roll of film is kept without any light sealing for a long time before processing. Bingo! Exactly my situation!
My workflow with film is kinda slow these days, and I tend to leave exposed rolls on a shelf as a reminder to myself to develop them. Otherwise I can forget about them completely. This small trick works only partially because while I’m constantly reminded, I still can’t get to doing it soon enough.
Now, when I learned the reason for those lines, I can at least make sure I keep my film light tight, and maybe I should come up with another way to remind me I’ve got some film to soup.