Factual error: The idea that anyone survived living long term in Pripyat Hospital is absurd. At the time the film was set the hospital was still radioactive and prolonged exposure by living there would be fatal. Nobody could survive on the grounds for more than two or three months without receiving a lethal dose of radioactivity. Incidentally, prolonged exposure to radioactivity doesn't turn you into a deranged zombie - it turns you into a corpse.
Factual error: The type of surgery shown made by the machine transversely cuts the rectus abdominis muscles. Besides not being a standard surgical incision in order to "remove a foreign body" as stated, it seriously damages important muscles of the abdominal wall making any immediate active mobilization of the trunk virtually impossible, even under strong analgesia.
Plot hole: The motivation of the cult in this film - a dark entity trying to birth their demonic god - is completely different from the cult of the original film, which was portrayed as fanatical Christian witch-burners. No satisfactory explanation is given in the film to explain this and in fact the film explicitly implies they are the same cult on several occasions. The explanation that the cult of the original film is some sort of offshoot or different sect of the cult from this film cannot explain this as it is pure fan speculation and conjecture (thus non-canonical) and in fact raises several more plot-holes.
Factual error: In the scene where Arthur first enters the house and starts going through paperwork, there's a shot of Nathaniel Drablow's death certificate which shows 'body was not recovered' in the box next to his cause of death. Such a thing would not be written on death certificates of the time - that box is reserved for 'Signature, description and residence of informant', a heading which can partially be seen as the camera pans across the certificate. For the death to have been registered, an informant's signature would have to have been there. (00:20:05)
Continuity mistake: Just before Steve Hadley is killed by the Merman, his tie is wrapped around his neck. When the Merman attacks him, his tie is worn properly. (01:19:40)
Factual error: At the start of the war, it says Fort Sumter 1861. However, on the battlefield we see the US flag with 35 stars. In April 1861, there were only 33 states.
Continuity mistake: When Alice knocks over the giant Licker in the Rolls-Royce, the driver's side of the car remains completely undamaged. Unlikely, but for the rest of the chase there are four giant scratches across the rear of the car. (00:51:10 - 00:52:05)
Continuity mistake: When Alex and her boyfriend are going up to the play gym, her hair style changes. It's crimped as they are walking up, then changes to straight when they are taking Hunter home.
Revealing mistake: Right at the end of the fight between Selene and the Uber-Lycan in the vampire coven, the Uber-Lycan throws Selene into some wooden panels, knocking her unconscious. As she travels through the air you can tell that she is suspended by wires; she travels far too slowly towards the wooden panels for someone who was just thrown. (00:48:35)
Revealing mistake: When Sparky's tail pops off there is a hole where it popped off. But when the tail goes into the bucket the hole is gone.
Continuity mistake: Towards the end as Taylor is holding the gun to The Corpse, the close up shots show the gun almost touching The Corpse's face. But further shots show it at least 2 feet away. (01:21:00)
Continuity mistake: At the battle of the bands Ryan's car is smashed by Tyler and his friends, specifically the windscreen and headlights. When Elissa drives the car minute later, it is in perfect condition. It's also in perfect condition when Ryan later reverses it into the garage.
Factual error: When the main character goes into the church, she looks at a picture from 1989 labeled "annual picnic". In the picture a boy (next to the main character's mother) wears a hat with a DC (DC Shoes) logo on it. DC Shoes was established in 1993. (01:00:00)
Deliberate mistake: Nearly every night scene inside the house is lit by very bright lights shining in the windows from outside. Yet every time anyone is outside the house, it's still just as dark. There is no light source from which these lights could possibly originate.
Audio problem: When Emily picks up the tooth from the box, she says "ew" but if you look in the mirror her lips don't move.
Plot hole: The killer has been living in the (apparently big enough) crawlspace between walls and floors, for over a decade. There is no explanation (or rationale) given on how he moved the various objects inside every room, closed doors, fed when the house was not occupied, hid the various corpses and, silly detail, concealed his own BO and the stench of his own living quarters, which is something hinted at when they find out his hideout. It is explicitly said that he is agoraphobic and does not get out, ever.
Continuity mistake: After Mike has set the tent up they go for a ride on their snowmobile to check the scenery out. They come uphill parking the snowmobile facing upwards. They get up looking at the scenery, and a few shots later the snowmobile has suddenly turned 180 degrees facing downhill. (00:13:40)
Other mistake: Aladdin slices off one head of the monster in the tomb. The body spurts copious amounts of blood and the severed head even bounces on him repeatedly, but Aladdin's shirt is immaculate afterwards. (00:04:40)
Continuity mistake: When Sheriff Cooper writes "Karsson" and underlines it on the chalkboard, the way it is written changes between shots. In one particular shot, the line he underlined it with is suddenly rubbed out.
Factual error: When the underground garage of the market is flooded, the car that Kyle and Heather are in does not flood. A car is not water tight like a submarine. Even though the water does not cover the top of the car, water would flood the entire car within minutes.