No mention of SQL, where NULL's behavior in the standard can lead to some quirky behavior that I've seen bite back in poorly designed systems. I've included a brief illustrative example. The expectation is that the UNION of two WHERE clauses, one using IN and the other using NOT IN should be equivalent to the same SELECT without any WHERE:

    WITH NullCTE AS
        (SELECT a.*
        FROM
            (VALUES (NULL), (1), (2)) a (Number)
        )
    ,One AS
        (SELECT Number = 1)

    SELECT *
    FROM NullCTE nc
    WHERE nc.Number NOT IN
        (SELECT Number
        FROM One)
    UNION
    SELECT *
    FROM NullCTE nc
    WHERE nc.Number IN
        (SELECT Number
        FROM One)

Running this will give you back a two-row table, containing 1 and 2, but the NULL is excluded from both WHERE conditions.

The naive expectation is that the combination of a condition and the NOT of that condition cover all possible circumstances.