Suddenly a tone got stuck on my model D. If I remember correctly, it kept on going simultaneously with tones played. With no idea what to do, I moved the Minimoog a bit, like shaking it sideways very little. The tone stopped, but the upper most C key popped up, about 1/4 inch.
I could, of coarse, open the Minimoog up myself, but it´s not in my home, and I won´t have access to it until next weekend. So, I´m asking you what you think might the problem. Replace bushings, is my first thought, but I can´t figure out how it caused the tone in the first place (the very sam C as the key).
I was going to suggest the A440 tone generator got stuck somewhat… until you mention the fact the stuck tone was the last key C.
I don’t know much about the D guts but it seems very strange to me that this tone sounded while other -normal- sounds were being emitted too…
Hopefully someone will chime in with better knowledge than mine 
Thanks for answering. I’m guessing the bushing got stuck somehow. Owned it since 2001, so it’s about time I take proper look in there anyway, perhaps exchange some of the worst springs as well.
From what I know, a blocked bushing would stuck the matching note. How comes you could hear others pitches at the same time? Or did I misunderstand?
As far as I remember it, that C sounded constantly, even when other keys were played. Weird and impossible, I suppose, but that went on for just a short while.
Took a look at the keybed yesterday, and half of the mentioned C key bushing still stuck to its place. The rest was small bits and pieces. So, new bushings are ordered, but the duophonic behaviour mystery remains.
Maybe you don’t want these new bushings. Maybe you do want a duophonic Minimoog 
I prefer the monophonic version, without a doubt.
New bushings, along with some contact springs, are on the way.