In various comments I've mentioned the problems inherent to mechanical rangefinder cameras. The complex connection between what you see in the rangefinder window and what you get on film is prone to errors. Most rangefinders aren't very precise. The few that are precise (like the Leicas) are rarely if ever accurate over the full range of focusing distances.
Even autofocus mechanisms have their flaws. My Fujica GA645 has two separate autofocus mechanisms, for near and far distances. The far distance focus is dead-on, but the camera consistently back-focuses by about 6 inches on near subjects. Fuji's repair center in New Jersey swears it checks out perfectly on their test jig. Now, when I photograph a near subject, I lean forward while focusing, press the shutter halfway to lock the focus, and lean back before pressing it all the way. Funky, but it works.
My friend Laurie Edison has been using one of the hard-bodied Fuji 645 rangefinders for many years. Those cameras are robust and reliable. A few years back, though, she started getting increasing numbers of out-of-focus photographs. It took us a while to be sure that there was an accuracy problem and that it wasn't just Laurie being sloppy. Eventually we became convinced there was a consistent back-focus problem. It appeared the camera was out of adjustment.
We tested this in a simple but effective manner. We sat down next to each other and I picked a brightly lit subject on the far side of the room that had very clear and contrasty edges. I focused and made careful note of the exact distance. Then I threw the lens out of focus, handed the camera to Laurie, and had her focus on the same point.
It took her quite a while to bring the superimposed rangefinder images into alignment. She kept hunting for the right setting. When she was finally satisfied that she had focused accurately, she handed the camera back to me. Her best focus distance was considerably (and repeatably) behind mine. She was having a lot of trouble aligning the superimposed images and that she was consistently back-focusing. There was nothing wrong with the camera.
Then I remembered something. Laurie had astigmatism, but she was not in the habit of using her glasses with the camera because she could see the rangefinder images sufficiently well without them, although it was a little blurry. Astigmatism doesn't just smear ones vision, it causes doubling of out-of-focus images. That's what was was messing up her focusing—the ghost images were creating a spurious and illusionary alignment in the rangefinder window.
New glasses with a better prescription for her astigmatism and a diopter lens for the eyepiece to let her see the image sharply with her glasses on solved the problem. She could once again focus fast, assuredly, and accurately.
So, the next time you think you may need to send your rangefinder in for costly repairs, first check to make sure you're not astigmatic.