There is a reference early on in the film by Brad Pitt where he says that one of his jobs was a projectionist and he would splice a single frame into a film that was showing, so that the viewing audience would see it but it would be so fast that they would notknow whether or not they had seen it.
Shortly after that you get to see huge cock that fills the screen almost as per his description - it is a split second shot.
You get to see it again towards the end of the film.
You have to be really watching for it or you will miss it.
Whilst we are on the subject of cocks in films - One of my faves is a Danny Dyer flash in an Easter egg on a DVD, I cant remember the film - but he was wearing White shorts and he just lobs it out.. Nice cock too