" 

 == Mass of composite systems == 

 Point 11: 

 ""...or write the energy-momentum relation m² = E² - p² and infer that, in a closed system, mass must be conserved because the energy momentum 4-vector is conserved."" 

 Yes, but we are still allowed to consider the mass of the closed system, which is not the same as the sum of the masses that are in the system. If you let an atomic bomb go off inside an infinitely strong container, then the mass of the container is the same as it was before the explosion. Even if there is no container, one can still define the invariant mass of the system after the explosion, which is not the same as the sum of the invariant masses of the particles after the explosion. That sum will have become less, of course.   "