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".
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.
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.