Revealing mistake: When Elsa is waiting in the lobby, she's wearing a dress which shows her normal, not wooden legs. It's very apparent when she is crying on the floor.
Continuity mistake: When Paul spits on the left side of Dandy's face you see Dandy get up and wipe the right side of his face. Also it appears the blood in his nose magically disappears between shots. (00:03:15)
Revealing mistake: When Dandy shoots Penny through the sheet, the second sheet flies out as it's hit with blood. But the sheet he shot through has no hole in it.
Other mistake: When Dandy goes on his shooting spree, the pistol he uses holds 9 rounds. When Jimmy returns to find everyone dead inside the main tent, there are 11 dead bodies laid out.
Chosen answer: Because on Halloween, the souls of The Dead are said to roam the Earth. That includes those souls that are trapped within the walls of the "Murder House".
THGhost