The current toon shader uses stencils to achieve its stencil effects. This effect has benefits when dealing with transparency. But it's sometimes less effective at rendering concave outlines or outlines that intersect with the mesh.

I'd appreciate being able to choose between stencil-based outlines and "traditional" outlines.

Here's a comparison between orels's toon shader and Poyomi.

Here's an example of outlines not being rendered in certain conditions: