CPU: AMD 860K
GFX: nVidia 960 GTX, 4 GB
OS: Xubuntu 16.04
Drivers: 375.82 from nvidia-375 (open source), installed from "Additional Drivers" section of settings window.
Blender: 2.79 2017-09-11 10:43, Hash = 5bd8ac9abfa
On the left, the plane has a principled shader. It only has a clear coat on it. No SSS, diffuse is RGB 0,0,0, no sheen, and no normal/bump mapping. The "Clearcoat Roughness" setting is at 0.0177 (and of course displays it at 0.018 unless you click on it).
However, the right plane is exactly the same, except the "Clearcoat Roughness" setting is at 0.0178, 0.0001 more. As you can see in the above picture, it's completely different, not just slightly different.
Here is the blend file:
Any questions, I'll respond as quickly as I can.