Press F12 in this .blend:
You may see the ground plane has two colors separated by a line:
Using the same Blender version 2.76b, but on a different computer, or using CPU instead of GPU, this line will not be present: the ground is one solid color. Also subdividing the plane or scaling it down will make the artifact disappear.
Our question is, what is causing this, is it a bug in Blender or a problem in the .blend?