At the beginning of the movie, the old man pulls a sled up near the barking dog and lays his ax on top of it. He walks out onto the ice, and when the dog is shown again, the sled and ax are gone.
When we see the security footage of the elevator and hallway, the time/date stamp says that it's February 14th, yet only a few weeks goes by before it's January 21st (as indicated by the website).
(at around 1 min) The main character runs into the bathroom, fills the sink with water, and puts his head in it. With the underwater camera, you see he gets his hair wet but when he pulls his head out of the water his hair is dry.
In the scene where Detective R. Rodriguez (Clifton Collins Jr.) is reading "the great day of his wrath has come" from his monitor to Detective A. Breslin (both are seating at their desks), Breslin's monitor is off, yet a few seconds later you can see someone's AIM buddy list on it.
When Aiden confronts Mr. Spitz about the pictures with Kristin, he is holding up a picture with just Kristin in it. The camera changes to Mr. Spitz then back to Aiden in which he is holding up a different picture, however his hands did not move.
When Corey checks his revolver all the primer pockets are empty and there is some light on the inside, meaning that the revolver is loaded with empty shell casings rather than cartridges.
In the scene where Taylor is suspended and looking at his brother killing himself, you can actually see the two hooks on either side of his head not move with the rest of his body.
Where this is supposed to take place in Detroit, when driving through the city at night about 20 minutes into the movie you can clearly see "The Bay" (Canadian department store) and "Rogers" (Canadian telecommunication company) signage.
At the part when Det. Breslin (Dennis Quaid) figures out his sons are in trouble and drives to pick up Sean there is a Canadian flag visible on top the the buildings in the background.
In attempting to identify the horsemen with their colors, Dennis Quaid's character makes a mistake. In Kristin's cell, he determines that Garrison is Pestilence when in actuality, Garrison is Famine.