The photo Warren shows is a real photo of Glenn Maynard and his school class.
Warren never serves soft serve ice cream to anyone in the film.
Warren's home tapes are a reference to the Bjork stalker's tapes - the final tape, in which he shaves his head and covers himself in boot polish, is a reference to the stalker's final tape.
The DVD and Blu-ray slick contains letters from a psychiatrist on the inside, set after the end of the movie, which explain what happened to Warren and also shed light on his background and mental state.