When Shawn discovers Talia has been kidnapped and finds her phone on the ground, the keyboard is open but when he does the recap accusing Hiltz Kooler of the kidnapping, the phone is shown closed on the ground.
When Shawn takes the final note, "Judgment Day," off the woman's heel, there is a black smudge on the note. But when it cuts to Shawn's point of view the smudge on the back is not visible.
In the scene where Shawn finds the note outside the convention center saying 'Judgment Day' an extra wearing a black shirt walks towards the convention in several different shots in the same scene.
When Gus is shown eating his pancakes, there are only a few pieces of pancake on his plate. But in one close shot there is a small pile of pieces that is not there at any other time.
When Shawn first surveys Malone's bedroom (at around 49 mins), the chessboard has a single soda can in the middle, the board is correctly oriented, and the chess pieces are in a (more or less) standard position. The next quick shot of the white pieces is the same. In the third shot of the board (at around 2 mins), a model of a dragon skeleton has been added to the center of the board, the pieces are randomly placed, the board is oriented incorrectly, and a different board is used (the brass plate on the front.)
A person being missing for 48 hours before police can investigate is a myth.
The opening scene featuring young Shawn is set in 1986, but one of the comic books his dad is holding is 'Checkmate' vol. 1, #28 by DC Comics, which had a June 1990 cover date. The other comic has an advertisement on the back cover for the film 'Tales from the Crypt: Demon Knight' which wasn't released until 1995.
When Shawn is on stage at the end of the episode he walks around, yet his voice is always amplified as if he stayed behind the microphones.