All gun safety rules are built with redundancy so that this doesn't happen even if a rule gets broken. You have to break at least two or three rules to actually hurt someone. Like, someone might fuck up at some point and flag someone with their muzzle, but as long as their finger is off the trigger, the gun isn't going to fire and nobody is going to get hurt.

One of the rules for an environment like a movie set -- the most important one, and by far the easiest to follow -- is that live ammo shouldn't be anywhere near the set at all. It isn't a unique one either, since similar rules are followed with many people's firearm storage and maintenance practices -- you keep your guns separate from your ammo unless you need to have both of them. Another one is that multiple people are supposed to verify what, if anything, is loaded into the gun, before handing it off to the actor. The actor is also supposed to follow the normal rules of firearm safety to the greatest extent that is possible for the scene. Everyone who was responsible for safety here failed at all of their jobs simultaneously, which allowed this to happen.