Consistent across 3 diffrent devices and operating systems.
Broken: v2.8 stable
still Broken: v2.83 Alpha, 9d90cad3ed39, 2020-01-17 00:13
Short description of error
Specific combinations of roughness and specular cause weird visual artefacts in cylces, that also look diffrent in contrast to eevee. It is possible for the edges of an object to look brighter or darker, than it's surroundings in a uniformly lit environment or an object appearing darker overall despite only roughness being changed.
Using all white background to better visiualize artefacts.
Exact steps for others to reproduce the error
Delete cube+light, set environment to be white color, add spheres, assign principled shader, try out diffrent spec+rough+metal values.
(Happens on both smooth and flat shaded objects...)
Here's a 2.80 file to try it out: