Edward Mordrake: Part 1 - S4-E3
Deliberate mistake: After Elsa kicks Bette and Dot off the stage, Elsa looks at the sheet of music on the stand and you see the name of the song 'Criminal' and it also states it's by Fiona Apple which is true, but this season is set in 1952, and Criminal wasn't released until 1996.
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