Steps to reproduce:
1. Open project repro on PC (Tested on windows 10 and windows 8.1)
2. Open Lightmap_home.unity
3. Turn final gather OFF in the Lighting window
4. Bake scene (Scene should look like PreFinalGather.png)
5. Turn final gather ON in the Lighting window
6. Bake scene (Scene should look like FinalGather.png)

- You can see correction more easily if you change the Scene window to Baked in the top right corner. Worst area is under the roof of the stands but occurs over the whole enviroment.

Reproduced: 5.2.4f1, 5.3.4f1, 5.4.0b17