Density functional theory (DFT) is increasingly popular as a tool for determining the electronic structure of molecules and materials.  The foundation of the approach is the existence of a universal and exact functional map between the ground-state density and the exact ground-state energy.  The limitations of the method lie in the lack of a systematic means of constructing this functional theoretically, compounded by the lack of direct experimental means to determine it.  One possible way around these limitations is the use of computational simulations using many-body methods like quantum Monte Carlo to probe and visualize key inputs into the universal density functional.  An important example is the exchange-correlation hole, the change in density about an electron caused by Pauli exclusion and Coulomb correlation effects, which is crucial to determining the electron-electron interaction energy; other useful quantities include local kinetic and exchange-correlation energy densities.  Visualization of these quantities can provide qualitative intuition and quantitative information for evaluating approximate functionals, but more importantly, it may reveal surprisingly simple conceptual “bugs” in their construction – and the insight into the underlying issues needed to diagnose and “debug” the problem.  In this talk I briefly address two such bug-fixing stories, one involving the correction of self-interaction error in the correlation energy of few-body systems, and to the construction of orbital-free kinetic energy functionals potentially useful for describing warm dense matter.