Win 10, GTX1080, i7-7700K 4.20Ghz
Blender 2.79, 2.78c, Cycles
See attached file. There are baked particles included. Hit render. There's a clear shadowy artifact across the mesh in Cycles render. The problem does not appear in Cycles preview.
The problem is gone when layer with particles is hidden. The problem is also gone when motion blur is turned off.
When I try to render on CPU, Blender crashes.