As the film begins. Mary Shelley's book Frankenstein hasn't even been published, placing the time as before 1816. And the time of the story is even earlier. Yet Pretorius opens the crypt of a girl who died in 1899, and connects Henry with his kidnapped wife via a "strange electronic device" (a crude telephone} that has to be 20th century. Most of the costumes - men in suits and hats - look straight out of the '30s.
The film is a direct continuation of Frankenstein, yet in that first film, Maria's father is named Ludwig. In this film he is named Hans and is played by a different actor. (This wouldn't be an issue if the actor who played Ludwig wasn't clearly shown in the recap of the first film.)
In the prologue explaining what happened in the first Frankenstein, a man is shown in close-up being strangled by the monster; however, the monster's sleeves are torn and his arms already burned by the windmill fire. Clearly this close-up was newly filmed and inserted as if from the 1931 movie.
After Dr. Pretorius shows a queen, a king, an archbishop, a devil, a ballerina, and a mermaid that he has created and placed in jars, a rear view of the table on which they are sitting also shows a jar with a baby in a high chair, who is not in other shots.
At the end of Frankenstein, the injured Henry is taken back home and resting in bed when the film ends. Yet at the beginning of this movie (following the Shelley/Byron prologue), Henry is still at the windmill and wounded. It is possible these scenes take place prior to the ending of the first movie, though this seems unlikely seeing as how this is supposed to be a direct sequel.
As the blind man prays over the monster, he clutches the monster's hand in his own and holds it to his heart. In the wide shots, the tangle of hands is near the top of the man's chest, right under his chin. In the close-ups of the man praying, there are no hands visible.
When the two hunters first discover the Monster in the hut with the blind hermit, one of them has a rifle and begins fumbling with it in an attempt to shoot, but drops it when the Monster knocks him down. He also loses his hat in the tussle. The hermit intervenes, but the hunters tell him that his 'friend' is a murderer and the Monster, hearing this, moves toward them again. The hunter then inexplicably has his hat back on his head and the rifle back in his hand and is hurriedly trying to cock it. As the Monster closes in, the camera pans back, showing the hunter now without his hat and rifle again and instead hiding behind a column. The quick scene of the hunter hurriedly trying to cock the rifle should be placed earlier in the sequence, when the hunters first appear in the doorway, and before the Monster knocks the hunter down.
In the wide shot of the Monster walking toward the hermit's cabin, in the bottom right hand corner of the screen there is a tree branch that the Monster brushes against. There is a jump cut, and the Monster is closer to the cabin, and the tree branch suddenly disappears.
This is a direct sequel to Frankenstein, yet Henry's father, Baron Frankenstein, who was featured prominently in the first movie, never appears. A character's dialogue briefly implies that Henry is the new Baron, which would imply the death of his father. But if his father had passed away, it would have been a major plot point or given some emphasis. Furthermore, he was alive and perfectly healthy at the end of the last movie, and was never attacked by the monster.
When the monster is being chased by the mob, (before they can catch him), he rolls a heavy boulder off a cliff, on them. The boulder is bumped by one of the villagers, and moves easily, showing it to be probably nothing more than a large ball of papier-mâché.
The goof items below may give away important plot points.
When the castle is self-destructing, Doctor Frankenstein can be seen against the far wall. Yet he is next seen outside in the arms of Elizabeth, watching the explosions. See also the trivia entry for this film.
When the Frankenstein Monster kills Karl and throws him off the parapet, the Monster's outline shimmies, indicating the primitive (yet effective) matte photography which placed the stormy sky behind him.