Is copy and paste a net time saver?

Many programmers have advocated disabling copy and paste because the resulting errors can be so painful.   That indeed was the source of this lack of total internal reflection in the little cube from an earlier post:

The photo on the left has a strong internal reflection at the bottom of the cube

I tracked a bunch of rays and it turned out the total internal reflection was occurring, but the polarization transformed wrong.   This code:

            r_s = 1-r_s;
            r_p = 1-r_s;



Should have been

            r_s = 1-r_s;
            r_p = 1-r_p;

Such typos are often so deadly because they take things in range (0-1 in this case) and keep them in range.  Fixing this at least has hit that most glaring problem (the sphere looks a bit better too):