A phrase or reference borrowed from the author's mentor, slipped in as an easter egg somewhere in the manuscript as a homage to that mentor. An example, from my own work - In one novel I have a Chapter called The Battle of the Somme. The chapter is not about the Battle of the Somme. In fact, it describes going through the Infiltration Course. However, the Battle reference is not only metaphoric, it's a mentorial homage to Tolkien, who fought and began writing The Lord of the Rings during the Battle of the Somme. (And for the Tolkien scholars out there, it wasn't actually LOTR, but The Last Cottage, which featured beginning of the Rivendell scenes). King does similar things all throughout The Dark Tower Series.
Edward C. Patterson