Continuity mistake: When Elsa reveals her prosthetic legs to Dot and Bette, they are two inches below her kneecaps, when in prior episodes they were just beneath.
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)
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