Silhouette edge

The easy example is looking at a cube in the direction where the face normal is collinear with the view vector.

The reason that this can be an issue is that a programmer might corrupt the original model by introducing the new silhouette edge into the problem.

Also, given that the edge strongly depends upon the orientation of the model and view vector, this can introduce numerical instabilities into the algorithm (such as when a trick like dilution of precision is considered).

After doing this calculation, you may notice indicator is actually the signed distance from the plane to the light position.

This is also the technique used in the 2002 SIGGRAPH paper, "Practical and Robust Stenciled Shadow Volumes for Hardware-Accelerated Rendering"