I once had a customer who bought a Leica M5 from my shop because he wanted "a really good camera for a trip of a lifetime to Africa", he came back a few weeks later complaining bitterly that the camera was faulty because all his pictures were blank, so I shot a film with it and processed it to test it, the negatives were fine, when I showed him the prints he didn't believe I had used his camera and when I questioned him about what his operating sequence was when using the camera I found to my dismay he had left the lens cap on all the time ! .