When Juliet finds out Shawn isn't the real "Mantis", Shawn tells her "I know, you know I'm not telling the truth. I know you know." That is actually the opening lyric to the show's theme song.
Spot the Pineapple: one can be seen as a salt shaker on the table when Shawn and Juliet are eating at the restaurant, one can be seen on the cover of the comic book, and a final pineapple can be seen at the Mantis' house.
After Shawn and Gus track down the Mantis, they ask about his motives. The mantis says that he was bored with his job which was mostly finding out where Rob Lowe had brunch or dinner. Gus, played by Dule Hill, quickly says candidly to Shawn, "Lucky's." This familiarity with Lowe quietly alludes to Dule's role on West Wing in which he played 4 seasons with the actor.
Since this storyline deals with a comic book character, at each commercial break the picture is changed into a graphic drawing and pulled back to be shown as part of a comic book page, with the other panels revealing things that happened just moments before. The theme is also done in comic book format with the cast turned into graphic drawings as superhero-type music plays.
When Shawn accuses Officer Reynolds of being The Mantis, he says, "Mantis Reynolds," which is a reference to Danny DeVito's character Frank Reynolds on It's Always Sunny In Philadelphia and his alias "Mantis Toboggan, M.D."