In Prob 1.2 of Griffith's "Introduction to Elementary Particles" 2nd ed., it says that:
However, I don't understand why it states that "they must temporarily violate the conservation of energy by an amount $mc^2$ (the rest energy of the meson)". If my understanding is correct, the "missing" energy was just temporary stored in the meson while exchanging, and therefore the energy is still conserved if we consider the system including the 2 interacting protons and the meson exchanged. So, is the energy conservation law really violated in this case? If true, where and why it is violated?