On my commute into work this morning, I listened to a fascinating programme that was on Radio 4 about three weeks ago, The Honest Musician's Fear of Accidental Plagiarism (sadly no longer on iPlayer) [Edit: Available again on iPlayer from that link!, 29 October 2015]. It discussed the question of musicians inadvertently plagiarizing others' material. It began with a great story told by Guy Garvey from Elbow about how he accidentally stole one of his best lines, "Oh, kiss me like the final meal", from another song, and about how he confessed to the artist who charged him "one beer" for the offense.
I began to wonder how far this is a problem in our area. Are we ever involved in "accidental plagiarism"? On one occasion I used the line "that with a sharp enough scalpel, everything is unique" in what I thought was an echo of a thought of Michael Goulder's. I later found the practically identical line in his work -- "With a fine enough scalpel, everything is unique". In cases like this, I think it's a bit closer to homage than plagiarism, but it is a fine line. And I bet I've done it on other occasions without noticing.
I have noticed a large number of occasions where scholars have borrowed actual phrasing from James Robinson's brilliantly told stories of the discovery of the Nag Hammadi tractates. One of the lines he uses, "the ultimate act of blood vengeance", is found in many retellings of the story, sometimes with minor variations like "quintessential act of blood vengeance" (Meyer) or "extreme act of blood vengeance" (Ehrman), but more often just as it is in Robinson. The difficulty is that the influential telling of the story is so powerful that the wording can stick.