Factual error: In the big battle scene at the Home Mart store after Robert turns the power off to the building, you see Robert grabbing welding canisters. When Robert has Ralphie turn the power back on, it cuts to one of the bad guys in the break-room, and a digital microwave starts counting down. A digital microwave could not have any memory of any prior cook settings, let alone any way to set it, since the power was already out. (01:55:40 - 01:56:35)
Factual error: In the diner scene, after killing the gunman, Robert shoves a single knife into an electric outlet, shorting out all the lights. To short out any circuit you must cross metal between both positive and negative wires. Also, the lights wouldn't be on the same circuit as the outlets. He wouldn't short out the entire store.
Other mistake: All the power is down but the fans in the bakery are still working.
Continuity mistake: When the guy robs the cashier at the store where he works, the guy leaves the store and there is a car waiting by the curb. He gets into the passenger side and you see the car drive off. But when it shows the back view of the car as it is driving away, there is only the driver and no passenger.
Continuity mistake: When he visits his friend in the hospital, he passes room 6 in the first angle, which he hasn't yet passed in the second angle. (00:22:10)
Factual error: At the end of the movie we see McCall on the beach with the sun low on the horizon, then the scene changes to the restaurant, then back to the beach with the sun below the horizon indicating a sunset (backed by the next few clips of darkness outside). You would need to be on the water looking back at Boston (where this takes place) to see the sunset, and you would see buildings, not a clear ocean. You would see the sunrise looking out over the water in Boston.
Continuity mistake: In the beginning of the movie at the diner, the building to the left has doors, and trees but the last time McCall is in the diner the building on the left is different and has windows. The last shot is actually the real building so there was some CGI editing.
Continuity mistake: After getting the barbed wire noose around the bad guy's neck, you see Robert push bags down to pull the bad guy up. When his friends find him strung up, the bags are gone. (01:47:15 - 01:48:10)