Peter Mullan kept forgetting to limp during his scenes resulting in many continuity errors. These are noted in the director's commentary as well.
When Gordon is sitting in his van outside his house looking at the christening photographs, we can see in the background his wife Wendy and their daughter Emma both in the garden. We then cut to the garden and see Wendy picking up Emma and carrying her into the house. We then cut back to Gordon in his van looking at the photographs, just before he leans over to pick up his bag and leave the van you can clearly see Wendy through the window carrying Emma into the house for a second time.
When Hank starts finding coins in one of the tunnels below the asylum he is wearing gloves. Then when he finds the second coin only a few meters further on his gloves are gone.
A CVS brand baby wipe container is in clear view when we are introduced to Carusso and Mullen in the van. A few moments later the baby wipes container is turned sideways.
Beginning at 07:48 "All these ducts are gonna hafta be wet-stripped" it's quite apparent that no asbestos abatement professionals were consulted for their knowledge.
When Gordon seemingly cut's himself, he doesn't, he removed the hand before the blade even touched him.
At the very end of this movie there's a typo in the copyright notice. The word motion, as in motion picture, is spelled "motin".
In the documents Mike comes across Mary Hobbs diagnosis is D.I.D. In the time period of the tapes D.I.D was still labeled as Multiple Personality Disorder, it was not until 1994 they changed it to Dissociative Identity Disorder to accommodate the DSM-IV-TR (a mental disorder diagnostic guide).
When Gordon and Phil take equipment from the van (21 min into the movie), the cameraman and the crew are reflected in the beige car.
The crew is on a very tight schedule with a $10,000 incentive to have a two week job done in one week, yet Mike has ample time to listen through the whole nine reels of Mary's interview tapes.