One more fault

There are two vibrating systems in a stringed instrument. First we have the vibrations in the string. Then we have resonating frequencies in the body of the instrument. The two systems are connected by the bridge and affect each other. A frequency of the string can be slowed down or speeded up by the frequency of the body.

This phenomena can be shown by experiments and we can't do much about it. Fortunately the impact is so small that we probably can't perceive it!

Now and then I notice strange phenomena that I can't explain with other than this. For instance, sometimes the chime at the middle of the string (the first overtone) can be flat although the open string is perfectly in tune. (It ought to be a tiny bit sharp because of the inharmonicity.)

Interplay between these two vibrating systems is also the cause of "wolf notes". If the note you play is close to the body frequency the two notes may by interference alternately boost and cancel each other, resulting in a wobbling sound like the bleating of a sheep.