Other mistake: The news anchor in the opening scene noticeably flubs her lines at one point.
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.
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.
Continuity mistake: In the underground lab, Dr. Su offers to surgically remove the false memories he implanted in John's brain. He drills a half-inch hole in the right side of John's forehead, which leaves the right side of his face covered in flowing blood. John can't stand the pain, breaks free, and he goes on a rampage throughout the underground bunker, killing every UniSol in his path. As he fights his way through the bunker, the bleeding hole in John's head switches from the right side of his forehead to the left side, then back to the right side again. Also, in the last scene (an unspecified amount of time after the bunker brawl), John has no scar on his forehead at all.
Revealing mistake: When Three Finger is driving the truck to ram the car containing Angela's husband you can see round the eyes that it is a mask being worn.
Character mistake: When Michael goes to the antique shop, the sign out front is spelled wrong; it reads "anitques."
Continuity mistake: At the end, the heater is on a chair when Sally chucks the locket on to it. After the spirit has been banished the heater is now on the floor.
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.
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)