The Lunar Prospector (LP) Electron Reflectometer (ER) instrument conducted a series of measurements of the lunar surface potential in a variety of conditions. Occasionally, when the Moon was exposed to the terrestrial plasma sheet and in daylight, large, unexpected negative potentials (∼−500 V) were measured. In this paper, we compare LP ER measurements with one-dimensional particle-in-cell simulations of the potential above the lunar surface when the Moon is exposed to both solar UV radiation and the terrestrial plasma sheet. The simulations show that large negative potentials will be measured by LP ER due to the presence of stable, non-monotonic potentials. Implications of these measurements to other airless bodies in the solar system are also discussed.