Continuity mistake: At the very beginning of the film we see the Seminar Leader start to write eXistenZ on the board. We see him write the letter 'e', but then the camera changes and he starts to write the letter 'X'. Notice that the letter 'e' has changed from how he originally wrote it. Then when he finishes writing eXistanZ the camera switches position again and the writing has all changed the look again. Most noticeable by the crossing of the letter 't'. As we saw him write it, then cross portion was much higher than in the wider shot.
Continuity mistake: In the scene where Juliette Lewis is at her downstair's neighbour door, knocking on it, we see the hall empty, but right after, it's full of the trash bags.
Factual error: Whenever one or more of the mutated, super venomous snakes attack someone, you can hear a rattling sound because they evolved from rattle snakes. However, it frequently shows the tips of the snakes tails, and they are completely bare. How can they constantly make that noise if they don't have rattles?
Continuity mistake: When Maggie talks to Eddie Bo Smith Jr. (the Chicago cop) outside the meeting of "receivers", you can see the cop's breath when he speaks, but not Maggie's.
Factual error: The film takes place around 1830, but you can see a character using a Colt Navy 1851, which was made 21 years later actually.
Continuity mistake: During the scene when they are trying to bring Mr. Leniog into the rear of the door we hear them trying to open the door from the inside, shaking it rather violently. From the outside shots we hear the door rattling but it's not moving at all like it is from the inside shots.
Visible crew/equipment: Mini boom mike type thing appears in shot just as the real killer clown is walking onto the stage about to kill Kate. It is visible when he turns to face the 'audience' in the lower right hand corner.
Revealing mistake: When Tsukiko's neighbor has his eyepatch removed, the prosthetic makeup for his scarred face and eye looks incredibly fake and unconvincing, and you can tell his bulging eye is just a painted piece of plastic with fake skin around it.
Visible crew/equipment: When the housekeeper says goodbye to Shigeharu you can see the boom microphone on the right side of the screen. Next to the light hanging above the dinner table. (00:16:42)
Revealing mistake: When the wishmaster stands up, the camera pans down to the glowing red gem on the floor. You can tell a crew member is shaking the thin piece of "flooring" to make the gem move.