Continuity mistake: At the end of the film the Monster tells Dr. Frankenstein, to leave. The Doctor and his fiancee run off. The Monster grabs a lever and says "We stay, we belong dead." He pulls the lever which blows up the castle. If you look to the left of the screen as the roof is collapsing, you can see Dr. Frankenstein knocked out on the stairs. Seconds later they cut to the Doctor and his fiancee running to safety outside the castle. It turns out in the original ending Universal killed the doc, but it tested poorly with audiences. They probably wanted a happy ending so they did a new ending where the doc lived, but they could not afford to blow up that castle again.
Suggested correction: We see Henry Frankenstein leaning against the wall with his arms outstretched, bracing for the collapse of the watch tower.
Continuity mistake: At the climax, the hero's large, short-haired dog dashes down to the village. It is obviously male. After alerting the villagers and foiling the villain, "he" runs back up to the castle, only "he" is obviously female on the trip back.