A PO put a too-thin washer under the lock nut for the cam chain tensioner on a '76 CB550. No biggie, except s/he then couldn't lock the adjust because the nut was biting the end of the threads on the tensioner lock screw, rather than locking against the cylinders. In an attempt to hold the lock screw in place while cranking that nut down, s/he broke the slot at the end of the screw, and then stripped out the lock in the tensioner.
Then my friend bought it, rode it for three years or so with the cam chain tensioner unlocked, and passed it to me. I tried to adjust the cam chain, and ended up doing a top-end rebuild.
When it was done, I couldn't get the stupid tensioner to lock in place. It felt like I was fighting myself... tossed a second washer under it, and boom, problem solved. If the PO had done that instead of forcing it, he could have saved me a top-end rebuild and saved that poor bike a lot of pain.