Revealing mistake: As Reg fights with one of the baddies with the chainsaws, as the baddie take a swing with the chainsaw he misses and cuts into the wooden door. You can clearly see that a vertical cut has already been made in the wood though, to help the chainsaw get in and cut. (01:17:55)
Revealing mistake: When the Tall Man gets hit with one of his own spheres he moves his head a bit too much in the subsequent shot where the sphere is drilling into his skull, and thus reveals the sphere's blades are actually dull props that are covered in fake blood to make it look like they're impaled in his flesh.
Revealing mistake: In the mausoleum where the priest is killed, when the woman is thrown back she is too obviously on a wire. She goes up at an angle, bounces off the wall still flies up for a second. If you look very closely, you can actually see the harness stings briefly. (00:53:30)