When Ryan and Justin are looking through Mossi Kasic's file with Segars at the police station, one of the papers is a "Notice of Inmate's Release" form, noting the "INMATE DATE OF BIRH" (sic) as 07/15/2009 and the "DATE OF SENTENCING" of 02/08/1995, suggesting that Mossi was sentenced 14 years before his own birth.
When O'Malley and Chang get pulled over by Officers Segars and Jackson, Officer Segars notices O'Malley's sergeant stripes and salutes him as Sergeant. But right after that O'Malley introduces themselves as Officers, not Sergeant O'Malley and Officer Chang.
In the impromptu boxing match, Justin is punched in slow motion on his lower jaw/chin area. The next scene, he is shown touching a bruise on his face next to his left eye, nowhere near where he got punched.
When they are spying on detective Brolin from Annie's apartment rain can be seen on the windows during the inside scenes, but when they look through the camera there is no sign of rain outside, or on the building they are staring into.
When they go to the first party dressed as cops, they just pull two uniforms out of a box, without any indication they had worn them before. At the party, Miller's brass name tag says CHANG, a joke they run with for the entire movie. But O'Malley's name tag already says O'Malley. This is before he goes online to get all the extras to flesh out the uniform.
Throughout the film, Ryan and Justin are supposed to be wearing real bulletproof vests, yet half the time they're wearing the uniforms, it's pretty obvious that they aren't, as there's no extra bill in their chests, and the other half of the time, the vest constantly moves from under their shirts to over their shirts in between shots.
In the last scene before the credits start rolling, when Ryan and Pupa arrive at E3 to pick up Justin, the Dodge Charger they arrive in has heavily tinted windows. When the action proceeds inside the car, you can easily notice the windows have no tint whatsoever.
The cop cars in the film, even the "real" ones, don't have a cage between the front and back seat, which real police vehicles have in order to prevent "perps" from attacking the driver and causing an accident to escape.
When the two real police officers enter the sporting goods store, they both rack the slide back on their handgun. No shell is ejected. This means that there was not a round in the chamber, and as such, if the officer merely drew his gun and pulled the trigger, no round would have been fired prior to racking (sliding back) the slide to chamber a round. No officer would ever carry a weapon like this - it's essentially carrying an unloaded weapon.