2019-08-01: On mathematical folklore                        rak
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Every math student at some point runs across "folklore" results.
Paul Taylor gives an entertaining description of what "folklore"
means in a comment [0] on Andrej Bauer's blog:

    "This result is folklore, which is a technical term for a
    method of publication in category theory. It means that
    someone sketched it on the back of an envelope, mimeographed
    it (whatever that means) and showed it to three people in a
    seminar in Chicago in 1973, except that the only evidence
    that we have of these events is a comment that was overheard
    in another seminar at Columbia in 1976. Nevertheless, if
    some younger person is so presumptuous as to write out a
    proper proof and attempt to publish it, they will get shot
    down in flames."
            -- Paul Taylor

[0] http://math.andrej.com/2012/09/28/substitution-is-pullback/#comment-21991