Continuity mistake: In the scene where Renfield is locked up in the sanitarium cell, the camera first zooms in to the horizontal bars on his window. Some shots later, the camera shows the guard looking under the doorflap, and since the door is opposite the window, the shadow of the bars fall on it. The bars have suddenly changed to a criss-cross pattern.
[Renfield is being seduced by Dracula's brides.]
Renfield: This is wrong. Do you hear me? Wrong. This is...WRONG ME! WRONG ME! WRONG MY BRAINS OUT!
Dr. Seward: Your master is gone forever, Mr. Renfield. You are your own man now.
Renfield: I am?
Dr. Seward: Yes. No one will ever control you again.
Renfield: You're right.
Dr. Seward: Good. Come, Renfield.
Renfield: Yes, Master.