When Mel Brooks was preparing for this film, he discovered that Ken Strickfaden, who'd made the elaborate electrical machinery for the lab sequences in Frankenstein (1931) and its sequels, was still alive and living in the Los Angeles area. Brooks visited Strickfaden, and found that he had stored all the equipment in his garage. Brooks made a deal to rent the equipment, and gave Strickfaden the screen credit he didn't receive for the original films.

When Gene Wilder leans in to kiss Madeline Kahn goodnight in her bedroom, her last-second quip "No tongues" was ad-libbed by Kahn.

The shifting hump on Igor's back was an ad-libbed gag. Marty Feldman had been surreptitiously shifting the hump back and forth for several days when cast members finally noticed. It was then added to the script.

Gene Hackman ad-libbed The Blind Man's parting line "I was gonna make espresso." The scene immediately fades to black because the crew erupted into fits of laughter. Hackman was unable to repeat the line without laughing with the rest of the crew, so the first take was used. Hackman was uncredited when the movie was originally released in theaters.

The cast, and especially Mel Brooks, had so much fun, and were so upset when principal photography was almost completed, that Mel added scenes to continue shooting.

Continuity

When Helga lands on the bed, she is lying on the bed with no bed covers on her. When her parents enter her bedroom, she has bed covers over her.

When Frau Blucher is about to escort Inga and Dr. Frankenstein to their rooms, the candles on the candelabra she reaches out to pick up are all lit. In the next shot, after she picks it up, the candles have been snuffed out.

After the class is dismissed, we see Herr Falkstein get up from his seat twice.

In the dart throwing sequence, Frankenstein's darts on the board change position between scenes after he throws his first set, presumably to make it easier for the Inspector to grab them as a single bunch in the following scene.

When Frederick grabs at the door handle, it breaks into two pieces. In the next shot, the door handle is completely intact.



Factual errors

Frankenstein's castle is in Transylvania, even though in Mary Shelley's book it is in Geneva, and in Frankenstein (1931), it is in Bavaria. Transylvania is the location of Dracula's castle.



Incorrectly regarded as goofs

When playing darts, only two go out the window. Yet at least three darts are in each tire, plus others in the spare and the driver's helmet. This is almost certainly deliberate.

The candle that operates the bookcase door appears to go out momentarily, and then comes back on, before Frankenstein and the assistant go down the passageway, but it simply got caught in a draft and dimmed completely down before coming up again - normal behavior of candles.

When Gerhard Falkstein meets Dr. Frankenstein in his class, he refers to Baron von Frankenstein as his great-grandfather, whilst in the rest of the film he refers to him as the grandfather. However, it is stated later in the film (most notably in the deleted sequence "The Reading Of The Will"), that the Frankenstein whose will is being read is the Great-Grandfather, and the infamous Victor Frankenstein is his son, or Frederick's Grandfather.

Practically nothing in the movie's portrayal of Transylvanian culture or geography is accurate. It's a comedy, not a documentary.

When the criminal is being hanged, it's raining heavily. When he's buried, the gravediggers are shoveling dry, almost dusty soil onto the grave. When Frederick and Igor dig him up, the mud on their clothes and the coffin is soaking wet and water can be seen dripping down near where Frederick had been lifting. This may have been intentional for Igor's "could be raining" line.



Revealing mistakes

Flipped shot. In the chase scene in the woods, the Police Inspector's prosthetic arm, badge, and monocle/eye patch "switch" from right to left. His companion cradles his gun in his left arm. In extra footage on the Blu-ray edition, the same man is shown in raw footage, cradling the gun in his right arm.

As Frau Blucher escorts Inga, Igor, and Frankenstein into the castle, the horses whinny at the mention of her name. Cloris Leachman can be seen putting a hand to her mouth to stifle a laugh as the scene fades to black. (And to think she had the gall to chide Gene Wilder for doing the same thing on many occasions.)

Gene Wilder barely stifles genuine laughter at several points.

In the classroom Frankenstein sticks a scalpel into his leg. A thick pad is clearly visible under his trouser leg, covering the whole top of his thigh.

In the 'Puttin' on the Ritz' number, one of the electric footlights explodes, alarming the Monster. The light however, stays entire and is still on, and it isn't clear what it is that Frederick is stamping out.



Audio/visual unsynchronised

When Frau Blucher is playing the violin to soothe the monster, her fingers stay in the same position the whole time she is playing, yet the music changes even while her fingers are in plain sight.

When Frankenstein first opens the door to the monster-making room, the rusted handle breaks into two pieces, but when he throws them down, only one piece can be heard hitting the floor.



Plot holes

When the monster enters the blind man's home, the man tries to light the monster's cigar with a candle. It is never explained why a blind man should keep a candle lit on his table.



Character error

The brain "depositary" should read "depository." A depositary is a person who receives a deposit in trust while a depository is a place where something is deposited.

When Igor, Inga, and Dr. Frankenstein go to catch the monster for the first time, they try to inject him with a sedative. Before she injects him, she squirts a little out the top to make sure no air is in the needle, but when she sticks him with the needle, she does not inject anything. In fact, when she pulls the needle back out, she squirts more sedative onto the ground.

The only way Dr. Frankenstein could have known about the hanging in advance was if he'd actually met the condemned man on the train to Transylvania (in a deleted sequence that was a spoof of Mad Love (1935)). This in turn would mean that Peter Boyle actually first played the convict, Rollo, in the deleted sequence.

The Monster's opinion of fire changes frequently throughout the movie. Sometimes he is deathly afraid of it, at other times he sits for hours next to an open fire showing no reaction at all.

In his brief opening speech to the hostile audience of scientists, Frankenstein uses the word "incredulous" when he clearly means "incredible."