Bill Denbrough Can’t Write Endings
Although the book reveals that Bill grows up to be a successful writer of both novels and screenplays, It Chapter Two adds an amusing footnote to Bill's prestigious writing career: he can't write endings. This is almost certainly a nod to Stephen King himself, who notoriously struggles to stick the landings of his books.
Although Bill's proclivity for unsatisfying endings is a running joke throughout the film, the most stinging dig at his craft comes from King himself, in a surprise cameo as a pawn shop owner. During King's brief scene with James McAvoy, he sells Bill back his childhood bike, comments on how much money Bill likely makes as an author, and declines Bill's offer to sign his copy of one of his books, confessing that he didn't like the ending.