Sic Scenarios
Let us imagine that Amelia writes in an email to her friend Bruce, "The Pope is quoteable." (She is naturally speaking about Alexander Pope.)
This being a daring and original sentiment, Bruce quotes her on his own blog: "The Pope is quoteable [sic]." He provides appropriate attibution.
Now, Camille, being quite a fan of Bruce's blog, sees this marvelous turn of phrase, spelling typo notwithstanding, and wishes to share the quote with the readers of her own blog. She doesn't have access to the primary source (Amelia's email to Bruce), so she quotes Bruce quoting Amelia: "The Pope is quoteable [sic]."
Now I'm sure the ambiguity has lept out at you. It is unclear to Camille's readers where the original error is to be found. Was it Amelia's error which Bruce noted or Bruce's error (either by misquote or by not noting it himself) which Camille noted.
Now, let's take a variation on that. What if there had been no error, but Bruce thought, incorrectly, that "quotable" should be spelled "quoteable," so Amelia wrote, "The Pope is quotable." And Bruce quoted her, "The Pope is quotable [sic]." Now Camille, who is quoting Bruce, does she drop the incorrect [sic] notation or note, with a [sic] of her own, the error in what she is quoting, "The Pope is quotable [sic][sic]?"
And to be consistent with the niceties of quoting, if Amelia wrote, "The Pope is quoteable," and Bruce quoted, "The Pope is quoteable [sik]," then Camille would seem compelled to quote Bruce as "The Pope is quoteable [sik][sic]."
I quite forgive you, dear reader, if you find my preoccupation with [sic] to be quite nauseating.