[thyrsus]

A nit: the article states "A guitar string is a harmonic oscillator; no matter how soft or hard you pluck it, a G string is going to play a G note."

It's not that simple. I have a steel string guitar which, when a string is stuck loudly, will play sharp, then asymptotically approach the Hz it has when sounding its quietest. I hear it, and my digital tuner displays the effect as well. I speculate that when the the string is more perturbed (arced further away from straight), the tension increases more than proportionately to the displacement from straight.