I ran across this passage (in a generally very well-done brief), which inadvertently highlights an issue that legal writers — students, lawyers, academics, and judges — should keep in mind:
Justice Story likewise explained in his noted Commentaries that any law dispensing with the requirement that jurors “must unanimously concur in the guilt of the accused before a legal conviction can be had …, may be considered unconstitutional.” 2 Joseph Story, Commentaries on the Constitution § 1779 n.2 (1891) (emphasis in original).
Justice Story did indeed describe the criminal jury trial as requiring unanimity (though in a slightly less helpful passage). But this item was not in Story’s original treatise; rather, I found it in Thomas Cooley’s fourth (1873) edition of the Commentaries. (Of course, Cooley himself was an influential constitutional commentator.)
But then I looked more closely, and realized that Cooley didn’t write this passage, though he endorsed it. The original appears in the 1858 edition, and was likely added by the editor of that edition, Edmund H. Bennett. So Story didn’t “explain” it, and I wouldn’t even say that Cooley explained it (though he did not edit out Bennett’s earlier explanation). The quote does have some authority, both because Bennett was a lawyer of some note (though most of his accomplishments came after the 1858 edition) and because the passage in an edition of Story’s highly influential treatise — even when the passage was not Story’s own — might have helped influence legal thinking during the mid-1800s, including when the Fourteenth Amendment was ratified. But the passage shouldn’t be credited to Story, and any credit to Cooley has to be properly limited.
This is generally quite common in influential legal treatises; they go through many editions, and later editions include material written by people other than the original author. That’s fine when you’re just citing the work for its own value as authority. But if you’re citing the work because of the authoritative status of the person who wrote it, you need to make sure that the quoted material was indeed written by that person and not by a later editor (or even by an earlier editor).
UPDATE: Whoops, originally said “even when the passage was Story’s own” (in the second-to-last paragraph, between the dashes); I meant “even when the passage was not Story’s own.” I’ve corrected this accordingly.