I was learning the halachos of berachos on things that are ground up, and it seemed pretty clear to me that the blessing on chocolate should be ha'etz. I found discussion in some more contemporary poskim who all seemed to take the same approach - "Really, it seems like it should be ha'etz, but the minhag is to make a shehakol, so let's see if we can explain it." That's fine with me - I have no problem coming up with an explanation to justify the minhag. What I was bothered by was the conclusion of R' Shlomo Zalman Aurbach, who after trying several approaches to explain the minhag, does not find any satisfactory explanation for it being a shehakol. He leaves it as a צע"ג - needs major further investigation. But he doesn't conclude to reject the minhag. It seems that he still accepts that the proper blessing is shehakol.
For myself, I am fine accepting the approaches offered in other responsa and making a shehakol. What I don't understand is why R' Shlomo Zalman, for himself, did not reject the minhag.
Chocolate was just a convenient example, so I hope readers do not get stuck on the example. My question, succinctly put, is at what point, if ever, can an established, accepted minhag be rejected as mistaken?