Continuum predictions for a sphere released under gravity in a Newtonian liquid are not consistent with experimental observations when a particle comes close to contacting another particle or a containing wall. Under these circumstances, the continuum approximations break down. Irreversibilities are observed in thin-gap particle interactions. We hypothesize that when the sphere is falling away from the plane, a bubble or cavity should appear in the close contact region. Depending on the size, the density, and the roughness of the sphere, the irreversibility may be caused by roughness, cavitation, or both. The results of these preliminary experiments show excellent agreement with our cavitation prediction.