There's a long history of films made for children with secrets that only an adult could enjoy. Remember the very phallic castle from the little mermaid? Well, a movie fan found an X-rated Easter egg in Shrek, 20 years after the film was in theaters.
The scene is when the film's villain, Lord Farquaad (voiced by John Lithgow) is laying naked in bed. He asks his magic mirror to show him the princess he loves. When the mirror obliges, he lifts the cover of his sheets and peers down at his naughty zone to notice that his body has been "awakened".