Gaffes

At movie's end, when the detective departs; he doesn't retrieve his revolver.

When the boy enters the doctor's house in the first scene, the doorknob changes. The exterior clearly doesn't match the interior.

When Dr Beecher is in Campbell's lab with the other two scientists, he says all the animals except the vampire bats have died. Yet on the soundtrack are the calls of guinea pigs. This occurs a number of times in later scenes.