Factual error: The idea that anyone survived living in Pripyat Hospital is absurd. The uniforms of the doomed firefighters were stored in the basement and are so radioactive that anyone straying downstairs for more than 45 seconds has about a week to live. The ground and upper floors are far less radioactive but prolonged exposure - in other words, 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.
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: 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: In the first movie the "Welcome to Silent Hill" sign is on the right. In this movie it's on the left.
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: A second group of Crypt Eaters attack Bruce and he tries to swing the bat at them. Some Brainchild solders arrive and suddenly shoot the creatures. Suddenly the bat is gone from Bruce's hand and is nowhere to be seen. (00:52:40)
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.
Other mistake: The news anchor in the opening scene noticeably flubs her lines at one point.
Visible crew/equipment: When the 2 bad guys get the car you can see the 2 cameramen reflected in the car door. (01:10:00)
Revealing mistake: When Ray reminisces about having fought the Germans in WW2, a flashback is shown with him fighting in a bunker. He is carrying a No.4 Lee-Enfield rifle which requires the bolt to be worked after every shot to reload. However, he is shown firing two shots in rapid succession without moving his hands at all, revealing the shots to just be flashes added after filming.
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)