Over the past 2 decades, neutrino physics has progressed from the initial discovery of neutrino oscillations to the precise knowledge of all three angles in the PMNS neutrino mixing matrix. We may now be on the verge of discovering the last piece of the neutrino mixing puzzle: leptonic CP violation. To observe CP violation, experiments must make precise, differential measurements of the appearance of electron neutrinos and anti-electron neutrinos. This requires unprecedented control of systematic uncertainties, and, in particular, an understanding of neutrino-nucleus interactions that is beyond the capabilities of existing theoretical models. The resulting "neutrino energy measurement problem" will be discussed, as well as a potential experimental solutions to this problem.