When Maxwell drags Dr. Meirshultz down the basement stairs,
they are both wearing Dr. Meirshultz' glasses, even though they only had one pair of glasses to begin with.
The woman Buckley strangles in a field is not the same woman he picks up in Dr. Meirschultz's office.
When a cat's eyeball is popped out, the cat we see actually only had one eye, and a glass eye had been inserted into the empty socket before filming. It is easy to see it is a ginger cat, used as "stand-in" for Satan, the black cat.
When Maxwell is conspiring with Mrs. Buckley in the final scene, Mrs. Buckley is wearing a very different outfit than in her first scene. In over-the-shoulder insert shots of Maxwell, however, she's wearing the dress and hat she wore in her first scene and her light-colored fur stole is gone.
When the wall is broken down, the actor playing "immured dead body" on the other side can be seen pushing the bricks apart.
The fighting cats at the undertaker's and the dog and cat fighting on the street, have been tied together with string that can be seen.
When Maxwell takes the glasses off Dr. Meirshultz' (who is supposed to be dead), his eyes move and twitch.
Various times throughout the movie, the "corpses" are visibly breathing.
After the cat's eye is removed, a closeup reveals it does not have the distinctive pupil of a cat.
A crew member's shadow can be seen on Maxwell as he stands up in the first scene.
When Buckley grabs the sleepwalking Maria Altura, the boom microphone pole's shadow is visible on the wall, to the right of the scene.