Ah, I didn't realise it was with two different cameras, sorry for that. As someone has already mentioned earlier, it would take some serious improbability for such an effect to be caused by two different cameras.

My instinct is on the sticking as described by polyglot. Now, if you dropped those cassettes in sugary water, let them dry, and then used them, without ever remembering doing that, would it look like this?

Thanks for a good mystery story for the weekend. I'm off to the darkroom, but will be checking how it unfolds.