Factual error: The Sensory Deprivation Tank used in the film is an MRI machine. It has no doors and makes far too much noise to allow for sensory deprivation. (00:47:15)
Continuity mistake: When Larry is driving Kyle out of Darkness Falls, Kyle pulls a gun on him to get Larry to turn back to the hospital. The camera angle changes from a rear shot to a side shot a few times, during which Kyle goes from looking directly ahead without the gun raised to facing Larry with the gun raised. This happens a couple of times just before the tooth fairy flies in front of them and causes the car crash.
Continuity mistake: When the police are taking Kyle-as-a-kid after his mother's death he's stood by the car (looking fairly evil) with many bloody cuts on his face and neck, but the next shot of him sat in the car shows he has just the one tiny unrelated nick on his cheek he had at the beginning of the film before the Tooth Fairy's attack.
Factual error: When we see Kyle as an adult, images of his several pill bottles are flashed on screen. One of these says "anti-psychotic" and is later referenced by a police officer; the other says "Take every 4 hours for depression." Psychoactive drugs would never have their purpose on the bottle, to protect the patient.
Continuity mistake: When Kyle, Kaitlyn, and Michael are going down in the elevator, the floor-indicator light switches from three to two. Then ten seconds pass, the elevator jams, and the light is still showing floor two. Pretty pokey elevator that takes more than ten seconds to move from floor to floor.
Factual error: Towards the end of the movie when Kyle, Michael and Kat are all in the lighthouse, they drop a green lightstick which then breaks in two. However, in reality, these lightsticks are virtually unbreakable as they are made of hard durable plastic which is why they are used by survival experts, divers and the emergency services. They are non-heat producing and work on a biochemical reaction - another reason why they are so difficult to break is to keep these chemicals in.
Continuity mistake: Throughout the movie where light is used to keep the tooth fairy at bay, her "sensitivity to light" seems to change a lot. A light stick that generates no heat and throws a dim glow keeps her away, but she'll come up close enough to a hot gas lantern to be decently illuminated.
Factual error: Towards the end of the movie when the power has failed, the emergency lights come on. After about 10 minutes someone is heard to say that they only have a few minutes of light left. Emergency lights in this country (England) are required to last 3 hours non-maintained, I'm sure that in America the characteristics of the lights must be similar, e.g. the emergency lights should be on longer than 13 minutes.