It's overlapping objects / surfaces I believe - e.g. OpenVDB overlapping did not render correctly before, but does now (see the New Features video). For refraction this applies too, the updated version is more accurate, although this just results in refraction "looking different", most likely your eye won't say that one is right and one is wrong (though the new version is more correct). It can be seen with, say, a teapot where surfaces meet and overlap such as where spout joins the body, or handle joins the body. While you won't say the old version "looks wrong", the new version is in fact more accurate.
