Stupidity: When Lawrence begins turning into a werewolf in front of Dr. Hoenneger, his fellow doctors, Inspector Aberline and the guard, everybody just sits in their seats and watches the transformation instead of running for the door.
Other mistake: When Lawrence is looking at his wound in the mirror, it has slightly healed. Later, when Sir John visits Lawrence in the asylum, he shows the bite mark he got from the feral child to Lawrence. Since Sir John is also a werewolf, the bite mark should not be there.
Dr. Hoenneger: Now, we are all aware that Mr. Talbot has suffered quite traumatic personal experiences. He witnessed his mother's self mutilations. His young mind, unable to accept it, created a fantastical truth, that his father is to blame. That his father is literally a monster. [Turns to Lawrence.] But, your father is not a werewolf. You were not bitten by a werewolf. You will not become a werewolf, any more than I will sprout wings and fly out of that window.
Trivia: Makeup artist Rick Baker has a cameo appearance in this film. He's the man who whistles to his comrades who are approaching the gypsy camp then a few seconds later, gets killed by the werewolf.