I kind of view that something like a teen throws a party. His parents are adamant they will be 100% watching over the party. Next thing they know some kid is drunk off his



and falls out a window. They SWEAR no alcohol was in their home and then surprise....couple of the kids snuck some in and took off to the backyard. Obviously the parents can be held, just like HGR, but maybe they both thought they absolutely knew no booze/no live ammo.
In either situation unless the kids arriving were frisked (and trust me as a teen that never would have stopped me from finding a solution lol) and the people entering the Rust set weren't frisked, or have to pass by metal detectors and dogs, so really other than HGR and the whole upper portion of people making the film, making it clear NO live ammo, how would she know?