According to the law of conservation of mass, why is it considered a mistake to add calcium to the product side if it is not in the reactant side of a particular chemical equation

According to the law of conservation of mass, the masses of the products and reactants must be the same. Adding calcium to the product side is considered a mistake if it is not in the reactant side of a particular chemical equation because it is impossible to produce calcium from thin air (unless this is a nuclear reaction).