Steps to repro:

1. Open Unity;
2. Import attached 'ShadowyArtifactTerrain' package;
3. Open 'test' scene;
4. Select Main Camera gameobject;
5. In Inspector click on Y field, hold left mouse button and drag the cursor to move the viewpoint above and below the terrain;
6. Observe terrain surface.

Actual result:
Round shadowy artifact appears on terrain when moving the viewpoint (see attached gif video and actual.png).

Expected result:
Shadow artifact should not be visible while moving viewpoint (see attached expected.png screenshot).

Workaround is to turn off Cast Shadows option in Terrain Settings of Inspector's Terrain component.

Notes:
- Reproducible on both Windows 10 and OSX 10.11.4;
- Reproducible in: 5.4.0b15, 5.4.0b14, 5.4.0b8, 5.3.4p1;
- Also reproduces when moving viewpoint in the Scene view (by mouse scroll).