There are lots of little subtle secrets in the background of Fight Club, and I found ' em. I went through Fight Club and picked out freeze frames of 11 of the best hidden secrets in the movie. Enjoy. . Fincher spliced in a penis.
Tyler splices a penis into a children's movie and into Fight Club itself. From the commentary it seems that David Fincher needed to make sure the penis had black hair so as to avoid rumors that it was Tyler's/Brad's. Tyler Durden Fight Club Wiki.
During a scene in which one of the Fight Club members sprays a priest walking by, Yes, that is a penis spliced into the final frame of the film.