My Casio CDP-100 keyboard (of the piano variety) has been a faithful companion since college and has worked like a champ through multiple life stages; we are well into our second decade together. At some point several years ago, through what I can only assume was a chance encounter with some progeny of mine, one of the keys stopped responding. It was a G#/Ab on the lower side, so I wasn't too bothered. Typically I was playing rarely and simply, usually alongside said progeny, so it wasn't a showstopper.
Some time later, I discovered Magic Keys for the Meta Quest (like Guitar Hero for your piano playing in VR) and went to give it a try. It was as fun and rewarding as I hoped it would be, and I played piano more in one night than I had in years. The problem came when I fixated on learning a particular song - I don't even remember which at this point - but it had an Ab in it. At least at this version of the app (some indeterminate couple of years back), it was self paced and would wait for you to play the correct note before it would advance to the next one. Nothing I could do could convince the app to please just ignore the key and let us all move on with life, so I rage quit, put the Quest back on ice, and went back to occasional and simple playing.
Fast forward more years of ignoring my Ab and I wanted to learn a song in Db. Unfortunately, this key prominently features Ab and rekindled my formless rage. Fortunately, I also now work from home and have a much improved work-life-balance compared to prior years, so the time was fulfilled to make it right. Initially I tried to reason my way into the inner workings, but reality disabused me of that notion very quickly, and I turned to the marvelously clear Casio CDP-100 Service Manual.
Thankfully I had my drill handy in my office already or I likely would have reached for my manual screwdriver out of habit, because there were 49 screws (!!) between me and my goal.
Once inside, I started peeling back the switch membranes to take a closer look. They looked clean and normal, but once I made it all the way to the offending key, I saw a strange crusty substance. Initially I suspected some sort of battery-like corrosion, which didn't make sense here electrically, but shared a similar look. On closer inspection it was orange? And looked suspiciously like dried spit and goldfish. Nice.
A few rounds of elbow grease later and the switch contacts were looking nice and clean and tested well. Now all back together and I'm free to play Ab all I want, which is still not terribly often, but it's nice to have the option if I want it, you know? Here's to decades more with this workhorse of a friend.