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.
Continuity mistake: At the end of Dracula's dance with Mina in the ball, as he is about to shatter the mirror which has given himself away, he throws a table at it. Mina is shocked, but as the camera shot changes to one of the mirror and the table approaching it, we can see that Mina is looking perfectly calm.
Other mistake: When they are trying to figure out who the vampire is and they hold a ball and then they pull down a curtain and reveal a mirror it is obvious in the reflection that Mina has no partner and when they do their "finale" when Dracula is lifting her up it is obvious Mina has some other support that will continue to turn her after she is shown in the mirror while the vampire has no reflection.
Visible crew/equipment: When Dracula is climbing down to Renfield's window, a wire is visible moving against the side of the building above Dracula.