How about this (unrelated) myth:

A friend of mine claims that the commonly agreed upon knowledge that guitar strings go dead after being played for some time is a myth. He says that regardless of whether a guitar string is clean or dirty, it vibrates at the frequency you have tuned it to, and that's it. Obviously, a dirty string generates the same measurement by the oscilloscope as the squeaky clean string.

So my friend thinks that the perception that a dirty guitar string sounds 'dead' is pure placebo.

If someone were to stretch that analogy, one might say that there is no difference between various hi fi systems. It's all in our heads.

I've heard someone compared an ultra high end turntable side-by-side with the entry level turntable and could not hear any difference...

Discussssssss......