TIL that ignorance of the law doesn't always hurt

In many criminal law systems you are not committing a crime when your (possibly mistaken) view of the factual situation doesn't constitute a crime, even when what you are actually doing does constitute a crime, as long as your mistake is "excusable". For example, if you take someone else's items believing they're your own (e.g. because they are very similar in appearance), you aren't committing larceny. (PL ref: art. 28 KK, CH ref: Art. 13 StGB)

Similarly, if you believe that factual situation matches a situation where a justification (e.g. self defense, or higher need) would cause your action not to be a crime, it is not a crime even if you were mistaken, as long as the mistake was excusable (e.g. if you destroy a car window to reach a realistic looking doll which you believed to be an unconscious child, you aren't committing the crime of property destruction). (PL ref: art. 29 KK, CH ref: Art. 13 StGB)

What I learned today is that you are not committing a crime when you are excusably mistaken _about the law_. Obviously, definition of "excusability" of the mistake does lots of work here -- IIUC the standard test is (a) would a typical person similar to you in education and background suspect it's a crime (b) did you have an opportunity to learn/ask whether this action is legal. (PL ref: art 30 kk, CH ref: Art. 21 StGB[*])

I find this very surprising, because it's a direct contradiction of 'ignoratia iuris nocet'.

[*] the Swiss law narrows it down to people who didn't and couldn't have known that it's a crime; I'm not sure if that should be understood in the everyday literal meaning of that phrase -- the context in StGB seems to suggest that it's intended to be slightly wider, so I suspect that this phrase has some specific meaning.

h/t to @freemo who caused me to look up all those things