After Wilbur, Chick, and Joan Raymond arrive at the castle, Sandra enters the room and her reflection can be seen in a large mirror. She exclaims "Wilbur!" in surprise of added guests. However, in the reflection, her lips don't move.
Bud Abbott and Lou Costello play characters named Chick and Wilbur in this film, but occasionally address each other as Bud and Lou. This was out of habit because these actors usually used their own names for their fictional personalities.
After Wilbur knocks the bundles containing their masquerade costumes out of Chick's arms and tries to dance with him, Chick say's "Come on, pick up these bundles and get dressed." Wilbur strides out of the frame without his bundle but he has it in his hand as he strides into the next shot.
Talbot , Wilbur and Chick arrive at the Masquerade Ball and you can plainly see it's already evening (with full moon in the sky), which means that Larry Talbot should have changed into the Wolfman already.
The placard that Wilbur reads to learn about Dracula in the museum is pristine in long shot, but creased and dented in close-up. The close-up was apparently shot after Wilbur bashed the sign against Dracula's coffin in the long shot.
The title is sometimes considered incorrect as the characters never meet Dr. Frankenstein or his family, only his creation "The Monster", whom Wilbur calls "Frankie". However, it was explained in Bride of Frankenstein that the Monster is named Frankenstein after his creator.
The goof items below may give away important plot points.
At the end of the movie when Mcdougal ties up the boat, he ties it to the left side of the ladder. A few minutes later Wilbur and Chick get in and it's tied to the opposite side of the ladder. It has also reversed direction.
After being attacked by the Wolf Man at the masquerade ball, McDougal has a bandage on his neck the next morning when he is hunting for Chick Young, who he thought was the one who attacked him. Later that night, when McDougal finds Chick and Wilbur when the Frankenstein Monster is chasing them, his neck is not bandaged and has no visible injuries.