That's why I won't take one of my 300blk rifles to the range with a 223 if I'm going with anyone who I will be letting shoot my rifles. If I'm alone I'll take both out but am very careful and keep the ammo and mags separate.
It shouldn't actually be able to chamber a 300 in a 223 but if the bullet were to be pushed back into the case or if you were shooting shorter bullets like a 110gr there is a chance.
People need to be careful and pay attention to what they're doing, this really shouldn't need to be said when using firearms but some people are careless and if you add to that someone who is inexperienced you have a dangerous situation.
Be careful everyone