(Possibly intentional?) The handwritten spell (that leads to the creation of the oatmeal monster), written by Tugwell, varies every time it is shown. Even the kind of paper seems to be different in some shots. Most strikingly, it varies in handwriting style, boldness of the ink and the nature of the characters. In some shots, the third character looks like a Greek delta while in other shots it looks like a D. Another letter shifts from a [ to a C to an E and back again. The character named Lovecraft calls them runes, but generously speaking no more than half of the characters could possibly be runes.
At about 1:16 Bordon gestures Lovecraft out of the back seat of the car, he has a 1911 pistol in his right hand. The sound of the hammer being cocked is heard, however the hammer on the 1911 still down. A 1911 is a single action pistol and can only be fired with the hammer back. As a former LAPD officer Bordon would know this.
During the red rain, the gargoyle on Hackshaw's roof is pelted with rain and the hooks of his wings jiggle, as if made of rubber. But we later see that his flesh can withstand Phil's gunfire. Also, getting kicked in the groin affects him, though he has no visible testicles and the rest of his body can withstand gunfire.
The guide wire on the oatmeal demon is visible as it flies to attack the restaurant manager.
When the zombie falls at the end outside the housing development, the ceiling of the soundstage can be seen just above the roof line of the house.
When Phil is reading the framed article about his and Harry's arrest, there are spelling and writing errors in the print. "Ceaslessly" is used instead of "ceaselessly". And "sighted" is used instead of "cited".
The bedroom in Connie Stone's 1948 house appears to have a round smoke detector immediately to the upper left of the door.
Near the end of the movie, Phil tells Connie that she "missed the fuck that saved the world" but we hear "screw that saved the world".
When Harry is telling his plan to Phil on the way to Bona Vista, he uses the word "goddamn" but we hear "goldurn".
When Phil and Connie kiss viewed from outside his office window you can see the reflection of the large spotlight used to simulate the lightning visible on the window glass.