Here's a picture from my ray tracer:
It looks mostly find but if you zoom in on the top sphere in the pyramid (a glass one with a refractive index of 1.5), you can see that there are some dark pixels which shouldn't be there. These don't show up if I remove the glass sphere and they don't show up on any of the reflective ones.
I've tried replacing it with another sphere with a refractive index of 1 (so no deviation to the light's path) but I still get these weird artefact pixels (in different places though). Has anyone else come across the same problem while creating a ray tracer? I tried changing my maximum ray depth to be larger but it made no difference.