The law of conservation of mass which suggests the absoluteness of mass by stating that the mass can never be created nor destroyed nor changed even at nuclear species level and during nuclear processes(nuclear reactions, radioactive decays, etc.), is no longer founded. Of course, within some problem domain, the amount of mass remains constant-mass is neither created nor destroyed. This seems quite obvious, as long as we are not talking about nuclear species or very exotic physics problems and processes. When we move a solid object the object retains its shape, density, and volume. On the other hand, in another domain, the result of the law of conservation of mass is quite not the same. It is found that the rest mass of nuclear species is measurably smaller than the sum of the rest masses of its constituent protons and neutrons. Mass is no longer considered unchangeable in this domain. Besides, the law of conservation of mass is proved to be invalid in basic nuclear processes like nuclear decays and nuclear reactions. Therefore, the aim of this article is to discuss some problem domain where the law of conservation of mass is not valid and along the way to reveal that the absoluteness of mass is recently not acceptable.