If this was corrected, the movie's outcome would have changed drastically!
When McClain (willis) and Zeus (jackson) are climbing across the cable they cut a guard in half. You'll notice that the guard manages to fire a few rounds out of his Heckler & Koch MP5K before he gets sliced (which proves the safety is off).
A few minutes later, McClain gives the gun the Zeus and he asks how to use it. McClain pulls back the cocking handle. Here, a live round should have came out (an error, but not a big one).
Later, when Zeus enters the room with the terrorist, he points the gun at Simon (Irons), he pulls the trigger, but the safety is on, when earlier, the gun was ready to fire!