Occasionally (very often), my raytracer project will have some bugs.
As the code is all about outputting images, the bugs are often also graphical in nature.
In this case the diffuse shader, rather than tracing outwards in a hemisphere oriented to the surface normal, was tracing in a flat 2d disk shape oriented with the world Y-axis. Sometimes bugs bring a cool surprise.
Here is another one. I am not sure what caused this but it certainly looks broken. Somehow the wing of the angel has reflected at a much larger scale in the back..