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.
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.
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.