This Mistake Is Not A Mistake
I'm referring to the opening scene where Phibes dances with Vulnavia in the ballroom. She walks upstairs as he lowers the cage of bats down the hatch in the floor which Vulnavia (in a different outfit) takes and puts in the car.
People consider this bit a mistake. Vulnavia appears in two shots one after the other. How can she be downstairs when she was just seen going up? How can she change clothes so fast?
The answer: The woman Phibes dances with is not Vulnavia, but a vision of his dead wife Victoria in her wedding dress. Victoria goes upstairs while Vulnavia is already downstairs.