Continuity mistake: As Corso leaves Quinta Fargas, he barely dodges a speeding car aiming to ram him. Clasping his canvas bag tightly, never dropping it, he presses his back against a large tree. As the camera cuts to a close-up, the bag is suddenly a yard away from him, on the ground.
Continuity mistake: At the very end of the movie when Depp is fighting his employer, Depp falls through the floor with his glasses falling to the tip of his his nose. But in the next shot when his employer looks at him, his glasses are all the way up by his eyes. His arms are tied by the floor meaning he couldn't have done it himself.
Continuity mistake: There are four versions of the ninth engraving. Three are intentional, and one is a goof. In the three intentional versions, the female figure resembles the Girl, and is shown with her hand raised, in front of the castle from the film. The unintentional version (the original illustration from the novel, before Polanski decided to make the figures resemble the actors) shows an anonymous female figure with her hand down, touching the beast, in front of an unfamiliar castle which is on fire. The three intentional versions are as follows: 1) The "correct" LCF version found at the end of the movie, showing with the woman with upraised hand, castle matching the one the characters visit, well shaded, and a star/sunburst coming over the wall in the middle. 2) The "forged" LCF version, showing the woman with hand raised and the castle on fire. 3) The AT version, showing the woman's hand up and the correct castle unmarked, with neither flames nor starburst. The goof is visible in the library scene, when Corso compares the Ninth Gate with the reference book. In one shot the ninth engraving is visible in both books and it is the unintentional Version 1: the woman with her hand lowered, and a different style of building in the background. However, in the zoomed shot ~8 seconds later, the castle has changed to the one later seen in the film and on the genuine engraving, and the woman's hand is raised again. In most of the scenes where the ninth engraving is visible, it is the wrong castle/lowered hand version.
Continuity mistake: Corso has reached Paris, checked into the hotel, and is sitting by a desk in his room to examine the charred remains of the Fargas copy. Look at the tumbler of scotch in this shot. It contains almost an inch of whisky, with ice cubes, and it's standing directly on the wooden desk. Now, as the angle changes and we see Corso from behind, the glass contains very little whisky, no ice cubes, and is standing on a white napkin. It's also more to the right of the book than it was. We can also see several scraps of paper, and what looks to be crumpled tissue, that wasn't there in the previous shot. (01:02:05)
Continuity mistake: Near the end, when Frank Langella performs the (flawed) satanic ritual, he pours a ring of gasoline and ignites it, then kneels down to pass his hands through the fire. They digitally inserted the reflection of the fire ring in his eyeglasses for about 10 seconds. Thereafter, his glasses do not reflect the flames, even after he douses himself with gasoline and self-immolates.
Continuity mistake: While comparing the Kessler copy with the Balkan copy, the engraving with the chess board signed with LCF is white. In order to have the devil appear in the castle, Balkan needs all 9 LCF signed engravings. When Corso appears in the tower, the engravings laid down on the table contain a black chess board. It's the one the middle, and quite distinctive. (01:23:00 - 01:53:44)