Revealing mistake: When Mombi's headless body wakes up and begins to walk towards Dorothy, you can see a black, blurry haze around her neck in two shots, where an effect was used to remove it.
Revealing mistake: When Mombi's headless body awakens and sits up, in the first two shots it is easy to notice that her body is actually simply a puppet of some sort, as the motion is not very fluid and it looks like a mannequin. Once she stands up, she becomes an actress whose head has been removed with special effects.
Revealing mistake: When Dorothy steals the key from Mombi's cabinet, as Mombi's body picks up her head to put on, you can see a small error. As Mombi lifts up her head, you can see the bottom of the cabinet warp slightly for about two frames (slow motion helps but is not needed), revealing that the actress simply had her head through a hole in the bottom of the cabinet (to make it appear severed), and her shoulders bumped into the bottom, making it shake/warp for a second.
Answer: The fire made the authorities discover the abused patients they'd locked in the basement.
Brian Katcher