When using IBL there are noticeable artifacts for at the north and south poles of both materials and the skybox.  This is caused by directly using the Equirectangular HDR Images for the skybox and IBL sources.  These equirectangular images diverge at the poles (the bottom and top rows represent the north and south poles.  Rather than projecting this equirectangular image into a cube map, we opt to directly sample from the texture.  This leads to artifacts when we need to sample the the top and bottom points.  Here are some examples: