mrdoob/three.js — comment on commit d196b36c513d48ea184cc58ca6471a63519748cb
File:  line 0
By alteredq:

Interesting, yes, fake normals used for lighting are probably not good to use for exact geometry computations. Whole purpose of this flag was to have face normals which would look a bit more like vertex normals (and were bit cheaper to compute if we already had vertex normals). I was already tempted to remove it last time we messed around that code.