I encountered a bug when I use a specular layer on a specular material (maybe it happens with other materials that use random walk/transmission too, I am not sure). In my case, I use a specular material with random walk medium and a specular layer. The reflection is completely messed up on the "dark" side of the face. When I use the same settings from the layer on the specular material itself there are no problems. In my attached renders I put a light plane on one side of the model. You can see immediatly what is wrong.
Here is the illuminated side:
And here is the "dark" side:
I exported the scene and it does not happen in the newest standalone and no problems there:
I dont know if this problem was already addressed in a later release but I thought I'm gonna post it here anyway.