Other mistake: (Season 4, Episode 1) During the scene where the phone sex operator lady is having her voicebox operated on, you can see on at least three different occasions her eyes flinch and squint. Obviously, she was "awake" when this was filmed, but during a normal procedure such as this, she should be completely sedated and she would not be flinching her eyes.
Other mistake: In episode 5-21: "Allegra Calderello," Allegra discusses with Shawn and Christian the fact that the show-within-a-show, "Hearts and Scalpels," featured a character based on her, called "pussy lips" because Allegra had her lips reconstructed with her labia. However, in episode 5-1: "Carly Summers," Shawn and Christian referred to their one-time patient as "pussy lips," but the network censors refused to air the word "pussy," and the euphemistic phrase "lady cha-cha" was used on the air instead. So Allegra would never have seen a character called "pussy lips."
Other mistake: When Sean is on the phone with Julia, by mistake he accidentally calls Julia "Julian." Julian is the first name of the actor who plays Christian.
Answer: "All I Know" by Art Garfunkel.