Steps to repro:

1. Open Unity;
2. Open attached project and 'test' scene';
3. Notice that there is huge terrain in the scene, having length more that 17000;
4. Go to Window -> Lighting;
5. Click 'Build' button to calculate lighting for the scene;
6. Wait until lightmap baking process finishes (may take some amount of time).

Actual result:
Shadowy artifact now covers nearly all of terrain's surface (see attached actual.png screenshot).

Expected result:
Terrain should be rendered normally (without shadowy artifact) after baking its lighting. See attached expected.png screenshot.

Notes:
- Reproducible on both OSX 10.11.4 and Windows 10;
- Reproducible in 5.4.0b21, 5.4.0b20, 5.3.5p1, 5.2.2p4;
- Reproducible for both Realtime and Baked GI;
- Reproducible for setting huge value (more than 17000) to either Width or Length fields of Terrain Component in Inspector;
- Doesn't reproduce when setting value '16000' to either Width or Length fields of Terrain Component in Inspector;
- Reproducible for both flat terrain and terrain with modified heightmap.