I encountered this bug while testing the Freestyle module in conjunction with the new Blender code with Bmesh Ngons. What I at first thought to be a problem with the Freestyle rendering module actually turned out to be a problem which is found within the Blender core.

The bug can be seen in the attached images. It occurs in this file at the juncture between a 5-sided Ngon and a regular quad polygon. In a Freestyle render this shows itself as an unwanted line along a (by the code perceived) break in the mesh. In Blender it shows up as a visual break in the smooth shading.

The programmer T.K. who is working on the Freestyle module informed informed me that this is caused when the conversion of of Bmesh Ngons down to triangles sometimes produces 'degenerate triangles' which are not properly dealt with prior to the render.

T.K. further felt that it was important that I also report this into the normal Blender Bug Tracker which I am now doing.

This file does not produce the same results on all platforms. On my Windows 32-bit platform it happens all the time. Another user running a Linux 64-bit OS was unable to reproduce the bug from the same file.

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Blender version: 2.63 r46904
Windows XP Pro 32-bit SP3
Intel Q9450 4gig RAM
NVidia GeForce 9600GT (driver 285.58)