Adaptation Displacement: The novel is still fairly remembered today and on the required reading list of several high schools, but when Frankenstein's brought up in pop culture, it's usually this version. And those reading the novel for the first time knowing only this film are usually totally lost.
Harsher in Hindsight: In the opening sequence, Frankenstein and Fritz retrieve a body that has been left hanging from a gallows in a remote location. Aside from speculation as to the reasons for the execution, when the body is taken down Frankenstein says because the neck was broken, the brain therefore was useless. This suggests that the circumstances of the man's execution were such that the doctor expected the man to have strangled to death, which is one of the worst-case scenarios that executioners try to avoid.
The name of the hunchbacked assistant is not Igor, it's Fritz. Ygor is an unrelated character from the third and fourth movies, who had a broken neck and didn't really assist the Doctor (he's the main antagonist in fact). The name was transplanted to the assistant for the parody film Young Frankenstein, where it's remained ever since.
The Monster is not named Frankenstein. In fact he is assigned no name at all.
When the Monster confronts Henry's fiancee. While the scene is still fairly chilling and unnerving even by today's standards, right as she discovers the Monster behind her and screams for her life, the Monster lets out a snarl that... well, let's just say it gives the scene a healthy dose of unintended hilarity.
People who've only seen Young Frankenstein are certain to find unintentional comedy in the scenes of Frankenstein went for a close parody.
What an Idiot!: It's a sad lookout when the monster with the supposedly abnormal brain ends up being the most intelligent character in the movie.
The Monster has risen, and Frankenstein, the doctor and Fritz are trying to control him... except Fritz has a torch in his hand and the Monster is afraid of fire and getting more riled by the moment by its presence. You'd Think They'd: GET THE BLOODY TORCH OUT OF THERE. Instead: They let Fritz get even closer with the bloody torch, agitating the Monster further.
Fritz has been abusing and antagonizing the Monster, enraging him. You'd Expect: Doctor Frankenstein to sternly admonish Fritz to stay as far away from the Monster as possible. Instead: Fritz keeps screwing with the Monster until the Monster hangs him with his own whip. And then is completely berserk.
Frankenstein and the other doctor have subdued the Monster, and believe he may be dead. The other doctor has promised to dispose of him quickly. You'd Expect: They'd get some gasoline, find a clear patch, and immedately incinerate the Monster then scatter the ashes just to be sure, if for no other reason than to prevent any diseases from the dead body parts from a myriad of corpses. Instead: The other doctor places the Monster on the gurney again and decides he's going to have him some dissectin'. The Monster snaps his neck.
The Monster has escaped and comes across a young girl playing in a field. You'd Expect: That faced with a rotting, scarred, lumbering creature, she would scream and run with all speed to find a responsible adult (which, given the caliber of the adults in this film would entail her running straight into another movie...) Instead: She asks "would you be my friend?" and gives him a bouquet of flowers. He ends up accidentally causing her death by drowning.
The Monster is on the loose, and is believed to be coming towards the Frankenstein estate, where Henry Frankenstein is about to be married to his fiancee, Elizabeth, and may even be in the house. You'd Expect: Henry would make sure that at least an area code was between Elizabeth and the Monster, would not leave her alone, and certainly make sure there was an escape route for her in case of trouble. Instead: He locks her, alone, from the outside (ensuring she can't get out) in a ground-floor room with big, unsecured glass windows, perfect for the Monster to walk right through to terrorize her.