[SOLVED] alarming freckles on smooth triangles

Hey everyone,

I have my ray tracer set up to the point where it can correctly parse .obj files with vertex normals, but something's not right in my rendering of them. I can see smoothing working well along the surface and then on portions of it I see patchy triangles that seem to think the light source is someplace else.. I've checked my implementation of Moller-trumbore and it's correct as far as I can see. But the problem must be somewhere in the normal interpolation, right? All relevant tests from the book are passing, so I'm pretty stuck here.

Here's an example of a icosphere render-gone-wrong:

Thanks in advance for any help!

And if anyone has some time to kill here's my source code: github.com/alecches/raytracer