Strange shadows

This is my third question in this raytracing work, but there has been progress: P So, I am implementing a C ++ ray tracer for my object-oriented programming class, and so far I have implemented monochromatic spheres and planes with support for reflection and mirror shading. This is an example of what I did:

Now I am trying to implement common polyhedrons. I use a modified version of this algorithm to calculate intersections with an arbitrary polyhedron from nFaces () faces, each of which is contained in the plane defined by Vec Polyhedron :: point (int face) and Vec Polyhedron :: normal (int face):

Vec Polyhedron::intersect(Vec o, Vec d)
{
    int face = nFaces();
    Vec ni(0,0,0), pi(0,0,0);
    unit te = -1;
    unit tl = -1;
    unit t = 0;
    unit N, D;
    Vec v = d.normal();
    int facein, faceout;
    for(int i = 0; i < face; i++)
    {
        ni = normal(i);
        pi = point(i);
        N = ((pi - o)*ni);
        D = v*ni;
        if(D == 0 && N < 0)
            return o;
        if(D != 0)
        {
            t = N/D;
            if(t > 0)
            {
                if(N < 0)
                {
                    if(t > te){
                        te = t;
                        facein = i;
                    }
                }else{
                    if((tl == -1) || (t < tl)){
                        tl = t;
                        faceout = i;
                    }
                }
                if((tl != -1) && (tl < te))
                    return o;
            }
        }
    }
    if(tl != -1)
    {
        if(te != -1)
        {
            v = v*te + o;
            return (v + normal(facein)*0.000000000001);
        }
        v = v*tl + o;
        return (v + normal(faceout)*0.000000000001);
    }
    return o;
}

, , ( Polyhedron, , Cuboid) . :

, . ?