So when we state that a particular interaction is "not conserving energy", we are basically admitting that we didn't pick the system "correctly". Sometimes we don't have the knowledge to do so, or som...So when we state that a particular interaction is "not conserving energy", we are basically admitting that we didn't pick the system "correctly". Sometimes we don't have the knowledge to do so, or sometimes it's actually simpler to consider a system that doesn't conserve energy, but we should be aware it's a problem with the choice of system, not about the interaction or phenomena itself.