Origin: https://en.wikipedia.org/wiki/There_..._knowns#Origin

The unknown unknowns are subtle back stabbing that can be mitigated only by being humble in confidence with what we think to know.
Who's the philosopher who said "In know that I know nothing"? ...ah, yes, Plato.

Well, we have some limited knowledge and experience, so for example my long experience with electronics carved in me the attitude to be wary with what I see on the instruments in relation with what likely the reality could be instead. Nevertheless it is impossible not to forget in one occasion or an other some "unknowns", even when they are known.

That said, this forum is cool as I found interesting people like you, or that anyway like to share their experiences/experiments and knowledge, without being fussy.
On Reddit I've posted the same post on the "metalworking" sub, but it seems that there, there are hypersensitive people that don't grasp the concept of rethorical question as some have criticized the fact I've put a quesion in the title. Gosh.

I will look better at those instruments you made, now I am quite in hurry.

Cheers, Claudio.