Operating system: WIn 10
Graphics card: GTX 1080
Short description of error
Black artefact appear on flatter surfaces when mixing 99.9% transparent and 0.1% glass shaders. Glass should be rough.
Exact steps for others to reproduce the error
Make a cube, switch to Cycles, add material, add Glass and Transparent Shaders, Mix them together, put factor on 0.999, increase Glass roughness, depending on roughness a entirely black circle will appear on the cube when viewed straight on.