Why do some questions which have been closed as not constructive appear on the FAQ tab?
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Many closed as not constructive questions are closed just because it isn't the domain of SO as Q&A site.
However they may be useful for many users, and can be asked by many users, i.e. FAQ. As it was noted in a A_funs comment
IMO it's a good thing, that high-voted questions appears in a FAQ even if they are closed as "non-constructive". Frankly speaking, when I am searching for an answer for my question, I don't care is it in domain of SO or not, I just want to find a reference to the info that many people already considered useful.