These are self shadowing artifacts. They are the result of unaccuracies in the GPU and depending on the GPU's model and vendor, they will look different. There's not much you do about them. Try to avoid making a shadow caster a receiver too and leave it to the vertex lighting to darken the areas that are facing away from the light source instead. In Robombs, i had the same problem...and if you look closely, you'll notice that only the walls cast and receive shadows. Everything else is either a receiver or a caster, but not both. The walls suffer from the same artifacts but they aren't that bad because the walls a tiny and nobody looks closely at them during the action.