Steps to reproduce:
1.Open attached project "case_793553 repro"
2."Build and run" this scene
3.Notice black terrain patches in the corner of generated terrain (Also check attached pictures if you weren't able to find those)

Note: These patches disappear as you approach them. If you play in editor, there won't be any black patches. I suspect that problem could be with realtime lighting because terrain is generated with scripts during runtime.

Reproduced on: Unity 5.4.0b17; 5.3.4f1; 5.3.4p5.