When Shawn Spencer was watching Mary's video diary dated July 30th, Shawn says that it was two night before he died. However, in the episode in which Mary died, it was clearly established that the date was February 24th.
When Allison Cowley is leading Shawn and Gus in the room to meet Mr. Yang, the boom mic is briefly visible on the top right hand side of the screen.
When Henry investigates the noise outside his house he has his finger on the trigger of his gun when, as a trained police officer, he would have known not to do so until he was going to fire.