Here's one for the archives. Perhaps it'll save someone a bit of future heartache.
I recently received a new-to-me, but used, lens for my Mamiya 7. I had a half-used roll of FP4+ in the camera so figured I'd use the last frames as a quick test, setting to autoexposure and varying the aperture from min to max. All the speeds sounded the same, and fast, which struck me as a little odd, but it was a bright day...
I developed the film and, sure enough, the test frames were underexposed and I could see the increasing underexposure from closing the aperture. Hmmm. The seller said the lens worked, so I removed the lens and gently cleaned its electrical contacts with a pink pencil eraser. After that it worked properly. The contacts had appeared fine before cleaning, but there was obviously some crud on at least one of them that came off with the eraser treatment.
As a precaution I cleaned the body contacts too, as well as the other lens I plan to take with me on vacation in a week's time. The eraser now lives in my travel kit.