Blooper

After Bart throws the ball through the window glass, every later shot that has the window visible shows no hole or broken glass.

During the opening credits, Elsa supposedly is walking along the Hollywood Walk of Fame, and she looks at the stars for Roscoe 'Fatty' Arbuckle, Jean Harlow, Clark Gable, Marilyn Monroe, and Rudolph Valentino in less than one block. In reality these stars are stretched along Hollywood Boulevard for several blocks, and Gable's is on Vine Street. Also, Arbuckle's star has his name Roscoe on it, not his nickname of "Fatty".

When Bart throws the ball through the window, the sound of the glass breaking is too late after the ball is thrown.

When Elsa is speaking in Lylah's voice, looking closely, it can be seen that the words are out of sync with Elsa's mouth.

Elsa's (Kim Novak) voice during her tirade against Molly Luther is clearly not hers. It is the guttural deep voice of an older heavy smoker, something that cannot be imitated. This is consistent with the plot, which eventually reveals she really is possessed, but it's interesting that no one notices.