Windows 7 Pro 64-bit
Intel Core i7-4770K CPU @3.50GHz
ASUS nVidia GeForce GTX 760 2GB GDDR5 DirectCU Mini
Broken: (example: 2.70 47e9057
...also broken in Blender official 2.69 and official 2.70
Freestyle has a problem rendering Ngons in such a way that extra, unwanted lines are generated.
It appears to be a faulty subdivision of the Ngons during that part of the render process where the information is passed through to Freestyle for creating the strokes.
If the Ngons are first triangulted – either by hand or else using the triangulate modifier – then the faulty lines do not appear in the Freestyle render.
The images show the problem.
Image 01 shows the render with the faulty lines indicated.
Image 02 shows the mesh. The Ngons that give the problems are selected.
Image 03 again shows the mesh, this time with the Ngons triangulated.
Image 04 shows the correct render following triangulation of the Ngons that give the trouble.
Here is the blend file.