Around the time I finished high school, I developed a roll of film out of my SLR and found only a tiny strip at the bottom of each negative was exposed. It turned out that the SLR mirror was supposed to move up slightly and then flip up, and the pin that did the flipping had broken, so it made a noise but didn't flip, and was blocking most of the film. Perhaps I should have noticed the viewfinder wasn't blacking out fully, but I wasn't that alert.
It wasn't economical to fix, so in a rare good decision, I spent more than a replacement would have cost and switched systems, getting a used Nikon F, which had the reputation of being bulletproof. I still have it.