When Carly and Zeke are in the video room she takes his Walther PPKS 380 and fires a total of 14 shots, but the Walther PPKS only holds 7 bullets.
In the dinner party scene, Jack Landsford, played by Tom Berenger, arrives at the party with slightly shorter hair than when he leaves. When he enters, his hair is trimmed tighter to his head, while when he leaves, his hair is definitely a tad longer on the sides, back, and bangs, enough to notice that this much hair would not grow in a matter of hours. This scene likely had a re-shoot.
The shooter's clothes change between the last shooting and the time the cops get there.
During the initial wide shot, when Carly and Jack wrestle for the revolver, his right hand is grasping the revolver. Then the closeup shot shows it as his lift hand.
When Carly and Jack wrestle for the revolver, his hand is holding the body of the revolver and the cylinder when he is shot. The hammer cannot be pulled back, and the trigger cannot be pulled, if the cylinder of a revolver is locked in place. It cannot be fired.
When Gus and Carly are at the grocery store, Gus doesn't pay for his item.
When Carly and Zeke are in the video room she takes his Walther PPKS 380 and fires 3 shots and the slide is locked back meaning the gun is empty
When crossing the street, a cab stops, honks, and driver says "why are you walking here?" as a nod to "Midnight Cowboy."
Polly Walker's digital answering machine makes sounds of a tape transport machine as it intercepts a message.
In the opening sequence, during the fall from the balcony, a cable or wire is clearly visible.
Following Gus Hale's death, Carly finds a newspaper clip whose title is "Horror high rise claims third victim". Gus was the third, Naomi Singer was the second, but it is not revealed who was the first.