Other mistake: When Spencer is on the phone to O'Hara, his iPhone shows the standby screen, not the screen that comes up when you're talking to someone on the phone.
The Devil Is in the Details... And the Upstairs Bedroom - S4-E4
Continuity mistake: In one of the first shots showing Agatha's friend Lucy behind the police line, Gus is visible in the background talking before Shawn and Gus arrive at the scene. (00:03:20)
The Head, the Tail, the Whole Damn Episode - S4-E15
Visible crew/equipment: The scene where Gus is smelling for the first shark and Shawn is 'divining' it. As they round the corner at the dock, a camera and some randoms are visible down the alley for a few seconds. (00:19:00 - 00:23:50)
Continuity mistake: When Shawn gets in Gus' car by the Psych office, the seat belt he puts on keeps changing position with each shot.
Continuity mistake: When Shawn wants Gus to throw him the baseball, the people walking in the background behind Gus keep changing between shots.
Continuity mistake: When Gus takes the Doritos bag from Shawn and sits down, every time the camera is behind Gus, the back of the bag is facing the camera, and when the camera is in front of Gus the back of the bag is also facing the camera. When Shawn stands up the bag is suddenly between Gus' legs.
Extradition: British Columbia - S4-E1
Continuity mistake: When Shawn flashes back to remember where he has seen Pierre Desperaux, he flashes on a photo of Pierre in Lassie's apartment next to Ernesto Chavez (on the right) on Lassie's wall. Going back to Season 3, Episode 11, when in Lassie's apartment, there is no photo next to Ernesto's photo. (00:03:20)
Extradition: British Columbia - S4-E1
Character mistake: When Shawn is checking Ed's book of valuables, the crown that Shawn thinks Despereaux will steal, has a listed value of "950,000 - 1,000,00,000 USD." I guess it's really, really(!) valuable, enough that a whole new numbering system was made up for it. (00:30:55)
Chosen answer: "Psych: The Musical" was supposed to air during the regular season 7 schedule, before Chief Vick is suspended and Jules discovers Shawn's lie, but the network decided to push it back to air as a special episode, sort of as a tie in with the holiday season and a lead in to season 8, which was to begin only a couple of months later.