Goofs

At least four different cats are used in the three almost identical hissing cat scenes. In the second scene, the cat changes into another half way through.

Although the Frankenstein Monster has been moldering in a warm, moist environment for years (in the muddy caves at the base of the cliff), his clothing is apparently immortal, too, for in the very next scene after his discovery in the caves his clothing appears pristine and brand new, showing no signs of the inevitable mold and rot that ordinary fabric would incur in such conditions (unless, of course, the Doctor has re-dressed him in identical clothes, which seems unlikely). In contrast, the clothing of Dr. Niemann, who was buried/and found with the Monster, has completely rotted away, to reveal only bones.

The skeleton of Dr. Niemann in the cave has a highly visible, perfectly even, horizontal seam line running all the way around the top of the head; it is obviously a skeleton model often sold by medical and education supply houses to schools, etc.

Some scenes of the Frankenstein monster use stock footage from Bride of Frankenstein (1935) and The Ghost of Frankenstein (1942) wherein the differences between Boris Karloff, Lon Chaney Jr. and Glenn Strange in the same role become apparent.

When the vampire bat is flying, the strings guiding it are visible.

When Dr. Edelmann drags Dracula's coffin into the sunlight and opens it, the rays from the sunlight are clearly blocked by the lid of the coffin. However, when the camera pans to Dracula, he is completely immersed in sunlight and is ultimately killed by the rays. Based on how Edelmann positioned the coffin, the lid should have blocked most, or all, of the sunlight.

In establishing continuity, the resurrection of Frankenstein is a well-treated plot element, but it is never explained how Baron Latos and Larry Talbot have come back from their deaths in that movie. However, the bullet fashioned by Ilonka was an amalgamation and not pure silver. So it can be theorized that Talbot was only wounded.