Other mistake: After they trace the number from the vampire bar they bring Marla's roommate in for questioning. Calling a bar, that has zero connection to the murder, and asking for blood wouldn't make someone a suspect or be enough to bring them in for questioning.
The Amazing Psych-Man & Tap Man, Issue No. 2 - S6-E4
Plot hole: The flashback involves Young Shawn and Gus getting ready to go to a comic book convention, Shawn in costume, with his father's consent. This contradicts the flashback in "Shawn Vs The Red Phantom," where Henry won't allow Young Shawn to even read comic books or play superhero with a towel around his neck.
The Amazing Psych-Man & Tap Man, Issue No. 2 - S6-E4
Other mistake: Before the first break when the scene morphs into a comic of the scene that just happened, Lassiter asks "You see him too, Guster?" in the comic on too is misspelled as "to".
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.