In the shoot out while the traffic is stopped and Ray Merrimen sets up the M249 (SAW) on the hood and pulls back on the cocking handle assembly, two sounds are heard.. Normally when a closed bolt weapon's cocking handle is pulled back it will make two sounds, the first would be the bolt being pulled back and a second sound when the bolt closes as it chambers the round. However the M249 is a gas-operated weapon that fires from the open bolt position, so there would only be a single sound made when the cocking handle is pulled back locking the bolt into the open position and it will only close when the trigger is pulled.
At the 1hour 14minute mark, Merrimen proceeds to tell Donnie to "always keep your finger off the trigger until you're gonna shoot somebody, if and when that happens, safety selector switch. One for semi, two for full auto." Earlier in the movie it was stated that Donnie did 1.5 years in the Marine Corps. With Merrimen being a former MARSOC Marine he would have no reason to go over that with Donnie. Aside from the significant amount of time being trained on the M16 platform at boot camp, Donnie would have attended either the Infantry Training Battalion or Marine Combat Training where he would have dedicated even more time to learning that weapon platform. Merrimen would have already known Donnie is more than proficient with that rifle.
During a final scene in the movie, Big Nick is seen, from a front camera shot, visibly holding his handgun with his right hand and supporting the grip with his left. However, when the camera cuts to an "over the shoulder" shot, the handgun is now being gripped in his left hand and supported by his right. This camera shot transition happens several times during this scene and each time the handgun switches hands.
When Big Nick asks for some Pepto early in the movie in the police station, the top of the bottle, including the pink liquid, is already visible on the desk. The other officer reaches into a drawer to get it and pretends to put it on top of the desk, but it is already there.
While LA has had the title of "The Bank Robbery Capital of the World," given in 1963 and it does have a history of infamous bank heists, it does not have a "robbery occur every 48 minutes" as it says in the opening title. Only in it's greatest year (1992) does it come close, with 2,641 bank robberies in the seven-county region of LA (1~ every 66 minutes).
Throughout the film, the license plates for the majority of the vehicles all share a close version of 4PCI*** with the last three numbers being slightly different. This includes several different armored trucks which, in reality, would bear commercial, not passenger, license plates. The marked police cars should display California Exempt license plates too.
High security facilities like the Federal Reserve would not allow a guest like Donnie (or any food deliveryman) to walk around unescorted. If an employee ordered food, they would most likely have to come down to the front desk and pick it up themselves. Either that or the deliveryman would be escorted by security the entire time they are in a secure area.
In the final shootout, gunfire is exchanged lengthwise along cars stopped in traffic. All bullet holes are neat and round, even in hoods and closed doors. This indicates the bullets passed through perpendicular to the metal, i.e. from right above or across the cars. This is obviously not the case.