I only teach things I can play well. That means to professional standard (BBC radio performance, for example). It is a simple rule which has served me well.
Recently I've been giving flute lessons to a woodwind teacher who taught flute to diploma level. It soon became clear that this teacher literally wouldn't have passed grade one. They have now decided to be a specialist in their own instrument and related (reeded) instruments and so have discontinued lessons. The finality of the decision reinforced by presenting me will all the flute teaching materials which now won't been needed.
I'm learning to play the classical guitar and have lessons an hour away - as that's the nearest teacher. One of my pupils spouses' wants lessons but cannot afford the time for a three hour round trip, and also, my teacher does not teach in the evenings. And so I agreed to some non-lesson shared practice time. I emphasised that in no way was I teaching guitar but could help with reading notation. I certainly shan't be charging a lesson fee.
I suspect this is how people slip into teaching instruments they can hardly play themselves?
