WIN8 - Geforce 980ti - (but the bug happens also on CPU)
The new reflection-fix (https://developer.blender.org/rBd6e769d32e7939e3bbd1986cdc4abd2b13135eab) introduces wrong shading in some cases (attached file)
How to reproduce
Just open the file attached and render, or preview in 3d viewport.
With pre-fix versions this doesn't happen, but of course we had black spots where the shader is self reflecting.
this is old 2.79a:
and this is the latest build with the fix: