In the film, the book "The Nine Gates" displays an inverted pentagram on its cover to represent its Satanic content. However, the inverted pentagram only became associated with Satanism and evil in the late 19th century due largely to a work by the French occultist Eliphas Levi. As the books were said to be in their original 17th century Venetian bindings, the appearance of the inverted pentagram is an anachronism.
When Corso and The Girl drive off in their "borrowed" red sports car, they are stuck behind a truck for a moment, and the car's brake lights are lit; however, the sound of the car accelerating and upshifting can be heard throughout the shot.
Real collectors and dealers wouldn't handle books worth $1,000,000 without gloves, and they surely wouldn't smoke or drink wine directly over them. Also, a book that old (not to mention the XVIII century Don Quixote volumes he takes at the beginning) would not resist the way Corso keeps chucking it in his bag or the fact everybody seems to be handling around. Furthermore, no dealer in his right mind would try to photocopy a four centuries old book by placing it in a commercial machine face down and pressing it as depicted in the movie: such actions would inflict severe damage to the printing and binding, drastically affecting its worth. Regardless those characters who see the book as a tool rather than a priceless collectible, Corso and the brothers Ceniza are experts in the matter, and would never treat such rare and priceless books that way.
When Corso describes Liana to Gruber, he says "Fortyish...", however, later on when Corso and The Girl lose track of the Liana's Rolls Royce, staring at the "St. Martin" sign, The Girl says to Corso: "Thirtyish..."
When Balkan and Corso enter the collection area early on in the film we see a building outside the many windows in the collection room. In the building outside the lights start going out one-by-one on one of the floors. But a moment later in the next shot, we see the lights back on.
When Corso and the Girl step into the lift at the hotel where Liana Telfer is staying, the Girl's hair is in a messy ponytail. When they get off the lift it's much neater, in the next shot it's messy again.
When Corso returns to the Book Sellers, and finds the missing page from the book as it falls from atop the old bookcase, in the first shot, it is entirely gray with years of dust, as it floats to the floor. When he picks it up seconds later, it is completely clean on both sides.
As Corso and The Girl were having sex in front of the burning castle, in the wide angle scene they were lying parallel to the castle's facade, but in the close shot, the burning castle is right behind The Girl.
As Corso was hastily picking up his stuff from the table in the library of the baroness, the postcard with the castle picture drops to the floor. In the next scene, he immediately turns and moves towards the burning room (without picking it up). But later, at the restaurant, he finds the postcard in his bag.
Inconsistency regarding the engravings that Corso claims to be genuine (signed LCF) and forged (signed AT). In an early scene he discovers the LCF signatures on 1) an engraving of an old man holding keys in his right hand and 2) a maze that has no exit. But in a later scene he says the genuine engravings show the keys in the left hand and that the maze has an open exit.
Even though Liana viciously bites Corso's upper chest during their post-sex fight scene, Corso's chest does not show any bite marks when he is seen shirtless in the hotel scene which takes place less than one week after the bite occurred.
While showing off his collection to Corso, Fargas presents a
"Dictionaire Infernal" by Jacques Collin de Plancy, which he says is a first edition from 1844. The first edition of this influential encyclopedia on superstition appeared in 1818, but its sixth edition, published in 1863, is prized for its illustrations, including 72 drawings of demons. The copy Fargas holds has a title page that is from the 1863 edition.
When Corso goes to the New York City Library for research, he pulls a copy of Books in Print from the shelf. All major libraries have used CDs of this reference guide for years. Also, he finds an illustration from the book he is researching. Books in Print never printed an illustration.
In the castle, when Balkan is about to perform the "ritual", he is shown to be pouring liquid out of a thermos into the cup/lid, however, as he finishes his pour the liquid appears to continue pouring into the cup, as if out of thin air. The computer-generated liquid is clearly out of sync with Balkan's physical pouring motion.
When Fargas is comparing both books for Corso, he supposedly opens both books to the same page to point out an error in printing. However, in the wide shot, it is clear the books are not both open to the same page. The book on the left has a very evident gap in one of the lines that the book on the right does not.
When Dean and The Girl are driving, and the sign that says St. Martin is reflected on the windshield, it is obvious that a computer generated reflection was superimposed over the windshield, because when the car turns, the reflection is jerky and unsynchronized with the car movement.
When Corso and The Girl get out of the red Dodge Viper near the Chateau, the car starts rolling backwards. The Girl moves back and reaches for the door to get in and apply the hand brake. The car stops rolling before she can take more than a step though so she just walks off like nothing happened.
The goof items below may give away important plot points.
Crew or equipment visible
In the fight between Boris Balkan and Liana Telfer on the altar, as they are wrestling, right before they fall over you can clearly see a wire pulling the brazier down. It runs from the bottom left corner of the screen to the top of the brazier.