Revealing mistake: Before The Monster sits all the way down on the teeter-totter that launches Helga into the air, she begins to get airborne before The Monster settles his end of the teeter-totter all the way to the ground.
Audio problem: When they first find the monster conscious, Frederick says "Is the sedative ready?" The reply, "Yes, doctor," comes from Frau Blucher, who isn't there, and doesn't make another appearance in the film until she releases the monster and he escapes. It should've been either Inga or Igor saying it.
Suggested correction: I thought it was supposed to be Inga.
Continuity mistake: When the blind man serves soup, there's a spoon in a cube on the left skewed to the right. A couple seconds later it's straight.
Continuity mistake: When the monster sits on the teeter-totter and flings Anne Beesley (Little Girl) through the window, she lands on her bed, head on pillow, eyes closed as if asleep (or knocked out by the landing). When her parents, rushing upstairs, open the door, she is tucked neatly under the covers.
Audio problem: When the Monster is tap-dancing to "Puttin' on the Ritz", the motion of his legs and feet do not correspond to the sound of the tap-dancing.
Other mistake: When Dr. F. is introducing the Monster in the theatre, he says, "as incredulous as it seems." "Incredulous" means unable to believe. The word he should have used is "incredible", which means unable to be believed.
Plot hole: Kemp, after visiting the castle, would've assured the people that Dr. F. Was not making a monster. So, why is that guy boarding up his house if he has no evidence that a monster even exists? Even if he did tell them that he believes that Dr. F. Was creating a monster, how did the guy know that it had escaped? Also, he says that he remembers the last time. He wasn't old enough to remember something that someone else's grandfather did.
Other mistake: When they are removing the casket from the grave, the grave is just slightly bigger than it. How did they get underneath?
Audio problem: At the end of the movie when The Monster is shaking hands with Inspector Kemp, he pulls his prosthetic arm off. Inspector Kemp is heard saying, "Oh shit," however his mouth says just "Oh."
Continuity mistake: The blind man reaches for the cigars, and when the angle changes, half a second later, the broken pieces of the mug have changed position and have also increased and suddenly appear scattered all over the table.
Continuity mistake: Frankie arrives at the door of the blind man, who welcomes him. Then there's a close-up of the monster from a bit far away where you can see his shirt and head as if he were alone; however, a frame later the blind man is right next to him, his head almost touching the monster's.
Continuity mistake: Inspector Kemp is known in the village to have had his right arm to be prosthetic (supposedly torn off by the original monster), but during the searching in the woods scene, after the riot starts, when he signals the villagers to keep quiet, his LEFT arm is suddenly prosthetic. The entire shot was apparently made of the villagers moving from left to right, but the negative was threaded backwards to get them to move from right to left. The Inspector's eyepatch is also on the wrong eye, and a villager is holding his shotgun left-handed (shown at the town gathering at the start of the riot as being right-handed).
Suggested correction: It's a running gag: Igor's hump changes sides, Kemp's missing arm and eye change sides.
Except that for the whole movie every time Inspector Kemp uses his right arm it keeps making a sound to indicate that it's fake, but he can move his left arm normally, showing that it's his real arm. Not only that but, the moving hump was Marty Feldman's idea. It can't actually be confirmed that Inspector Kemp's prosthetic right being switched to the left for that one scene was part of the gag or not.
