I was chatting with a professional musician the other day, and she said something that caught my interest. Apparently a really good violin will, if not played for a few months, "go to sleep." It's difficult to say exactly what this means, but apparently its tone dies, it becomes "less bright" and, to the professional ear, "sounds sleepy." If played again regularly and frequently for a few weeks it will then gradually "wake up" and become its former self.
Theorists occasionally doubt it and skeptics demand double-blind tests. Practitioners laugh at them. Many have experienced it "first hand."
I hypothesised that the wood contracts and becomes "tighter" when it's left alone, and that the vibration of playing will open up the texture. She hypothesised that the glue will expand while playing, and when left alone will tighten up.
However, I've looked around a little and it's apparently a well-recognised phenomenon.
I would never have guessed, and found it interesting.
Compare and contrast:
http://www.google.co.uk/search?q=%22software+rots%22
The best explanations I've seen involve crystallization of the varnish (rather like how bread goes stale via starch crystallization), but I don't believe there has ever been any conclusive scientific study done of this.