We survey Lunar Prospector Electron Reflectometer measurements from times when the Moon was located in the geomagnetic tail, in order to characterize the occurrence of large (>500 V) negative lunar surface potentials, as identified by upward-going electron beams. We find that charging to these levels is rare, but that such charging events do occasionally occur in both sunlight and shadow. Large surface potentials are found primarily in the plasma sheet, and their occurrence depends mainly on the incident electron spectrum rather than surface properties. Most examples occur in shadow, where the current from energetic electrons dominates. However, some occur in sunlight, suggesting the occasional presence of either lower photoemission than expected or non-monotonic potentials. Depending on the electric field scale height, significant electric fields of up to ~100 V/m may sometimes exist at the lunar surface.