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.
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 Henry visits Dr. Pretorius, Pretorius shows Henry his menagerie of homunculi. Pretorius says, "I have to keep my eye on the King." He then covers both King and Queen's glass jars with a dark cloth sheath. But after panning to the Arch Bishop's glass jar, the King and Queen are inexplicably uncovered again, whereupon the King climbs out of his jar and onto the table.
In the iconic scene in which the blind hermit is teaching the Monster a lesson, the hermit lights and smokes a cigar and, in order to overcome the Monster's aversion to fire, offers his lit cigar to the Monster so that the Monster can enjoy smoking the lit cigar (which the Monster does). Immediately after (with only a short lapse in time during which the Monster is also humorously seen gulping as a reaction to smoking for the first time), the hermit is seen with a second lit cigar in his hand (not the one the Monster has just smoked, which the Monster still has in his own hand when the Monster utters the unforgettable line "Alone, bad...friend good!").
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.
When the two hunters discover the Creature in the hut of the blind hermit, one of them attempts to cock a rifle, but a split-second later when the Creature strikes him the rifle has disappeared. A portion of the film was deleted (and ultimately lost) which showed the hunter accidentally dropping the rifle.
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.