Is conservation of mass absolute?

And all chemical reactions that have EVER been observed and studied in studied in detail exhibit conservation of mass; this a fundamental, underlying principle of all chemistry. What does this mean practically? It means that if I start with a $10 \cdot g$ mass of reactants (from all sources), AT MOST I can get a $10 \cdot g$ mass of products (from all sources). And because chemical reactions are seldom quantitative, and product mass is lost upon handling, in practice I am not even going to get that amount.
However, for certain nuclear reactions of very heavy elements, or very light elements, MASS LOSS can occur on fusion or fission. And given the famous relationship $e = m {c}^{2}$, the energy output from a nuclear reaction may be ginormous.