This kind of thing killed Brandon Lee during the filming of "The Crow"....Lots of real guns on the set. Lots of blanks. Too many hands in the cookie jar. A Director that wanted as "realistic" a scene, and live ammo introduced into the mix....
Ball was dropped, safety was compromised,....somebody dropped the ball....and folks were hurt,...and in Brandon's case,...killed.
Single point, as MB posted is a firewall. All guns check in, all ammo checks out,....not just once, but two or three times.
During the Miami Vice days,...the armorer would literally lock down the set if one blank, or spent case was unaccounted for.