I think it's one manifestation of what is nowadays technically called "aliasing," pronounced as in "forming an alias."

It happens when two wave phenomena are slightly out of phase (or, more likely, some integral multiple of one is slightly out of phase with the other), giving the result that one moves slightly ahead of or behind the other. That gives the illusion that a third object is present, with different properties/frequencies.

Any kind of wave will do: light (as in this thread), sound (think of tuning a stringed instrument using "beats"), radio waves. (I suspect anything based on interferometry works on this principle, but now I'm getting a little off base.)

Aliasing in medicine is seen in ultrasound imaging and must be tuned out, or at least compensated for.