This is not a bug, just fewer samples are taken when the rays bounce from the table. Corona currently uses NO adaptivity. Just imagine the speed after I add it :D
So, do I just need time to remove the noise? When you'll add the adaptivity ^____^' , like in VRay?
and, what about the shadows through the solid glass?
As you can see in my floor, now I use a solid glass with "cast shadows" option disable in the shader.