Our quarter inch plus and jacks have a basic standard shape, but they vary a little. From time to time it happens that the exact shape of a particular plug and the exact shape of a jack don't quite agree, even though they work everywhere else.
I had a run of molded plugs on Fender Hot Rod DeVilles speaker leads that were somehow just ever so slightly undersized and did not reliably push the shunt contact aside, so the speaker would cut out now and then. A gentle reform of the contact (fancy word for bend it) and it works.
My best story was a Crown DC300 amp with a row of fuse holders across the front. Mains fuse and V+ and V- fuses for each channel. One channel was missing V+. Sure enough, no continuity through the fuse. I pulled the fuse intending to replace it, but found it intact. A different fuse in that holder worked. And the fuse in question also worked in another holder. In fact all the fuses worked in all the holders except this one fuse in that one holder. SOmething about that one fuse that would not work in that one holder. I replaced that fuse anyway, so it would never find its way back into that holder, but really...