When Haversham alludes to Shawn being behind the haunting, he says that he heard the voice on the phone say, "Gus," right before the call ends. In the scene where Gus walks in on Shawn whispering into the phone in the attic, Shawn clearly stops whispering and hangs up the phone without saying anything.
When Shawn's mother is having her interview with Lassiter, she has her hands on her knee. They switch from being both hands on top of each other to hands beside each other as the shots change.
When Sean is looking in the basement window, Gus tells him good luck with "stachybotrys virus." Stachybotrys is a type of mold, not a virus.
The bush that Haversham leaps over while running to catch up to Gus in the woods is an obvious prop (it tips over and rolls down the hill behind him).
When Gus surprises Shawn in Mr. Haversham's attic, Shawn composes himself and clearly hangs up the phone before saying anything. Yet later, in in the office, Haversham says that he wised up to the whole scam when he heard Shawn on the phone uttering Gus's name in surprise.