As McClain is escaping from Ellie's apartment, a detective shoots at him, breaking the car windshield. The glass breaks in jagged shards, which is wrong. Car glass is tempered safety glass which breaks in beaded chunks. Safety glass has been standard in cars since the 1920s.
Marty collapses in an asthma attack. But asthma makes breathing difficult, sometimes even extremely difficult to the point that one must sit or lie down as one gasps for air. Asthma does NOT, however, hit a person like an epileptic attack. It does NOT cause one to immediately collapse NOR does it cause their eyes to roll to the back of their head.
McClain throws Negli (Sutherland) into the steam jet, who is killed almost immediately. Though one can die from heat stroke from hot water, one would have to be immersed in the steam (or hot water) in order for that person to have their temperature rise to a dangerous level. But Negli is NOT exposed the steam, let alone engulfed by it long enough to be killed by it.
At 9:55, as Jim Brown goes up the escalator, people can be seen staring at him and looking at the camera.
During the car chase scenes between McClain and Kifka, they are mainly on dirt roads, but tires are heard squealing - which does not happen on unpaved roads.
McClain and Brill drive off, and McClain's partners follow. Brill turns around and is able to tell that there are three in the car following him. BUT Brill has to be able to see into a car with headlights on and pointing at him. The headlights would cause a blinding effect and Brill would NOT be able to see anything behind those headlights.