Try plop rendering the areas with a higher quality setting (consider blurry transitions). Doesn't solve the why, but may reduce the effect. I notice that the drapes have a bump on them that has a linear element (there are lines on the drape above the right hand decanter sample) and I wonder if the bump mapping is having problems with the refracted light through the decanters. You could prove this thought by switching off bump on the drapes.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.