Well, the lucky part is the important one. What it boils down is that such system is either self-contained or has rigidly defined outside interfaces. For anything that deals with a physical reality outside of the pure computational realm this tends to be impossible. You are not going to build an entire warehouse to serve as the physical part of testing environment and even if you did so it will not be really useful, because the thing will be different than the production one due to who knows what tolerances involved in building a physical things. In same vein if you interact with external services you can either mock them or use whatever testing environment the communication partner provides, in both cases it is bound to not behave the same way as the actual production environment.