Audio problem: John is on the wing of the plane, questioning why he is there. His mouth isn't moving with what he is saying.
Continuity mistake: When Barnes and the SWAT team reach the Annex Skywalk, they get on the moving sidewalk. Before the terrorist stops the moving sidewalk by pressing the button, the sidewalk is already stopped, even though the light was still green at the time.
Factual error: There is no Glock 7, and Glocks are produced in Austria, not Germany.
Factual error: One scene has a shot of an aircraft, in flight, with a red light on the starboard (right) wing and a green light on the port (left) wing. Backwards.
Continuity mistake: When the general exits the plane with Colonel Stuart, nobody shuts the door. It is closed in the very next shot.
Answer: If you listen, he says something to the effect of "I know I had that guy in my sights", meaning he is amazed he missed. So he checks the bullets in the magazine to see if that's the problems. Blanks are simply shells with no bullet that are crimped shut to hold the powder in. The difference between blanks and live rounds is very obvious even at a glance.
Grumpy Scot