Win 10 64, Nvidia GTX 960 4GB
Broken: 2.79 38b72fb
Short description of error
When a texture with pure black is plugged to the distance input of AO node, it completely ignores the texture. Also when using Factor output of a checker texture it doesn't work even with values above 0.
Exact steps for others to reproduce the error
Attached is a blend file with a simple geometry and AO shader. Just render preview in viewport. I added two colors (as "Combine RGB" for better readability of values) - one is pure black 0,0,0 and one is slightly lighter 0.001. Factor output doesn't work at all.