I recently set up a (very basic) ray-marched path-tracer, and I'm getting strange surface patterns on my area light [link]. I'm using known-valid analytical normals for the light's surface, and the same patterns appear when I use a well-tested gradient estimator anyway.
Ray bounce directions are calculated with amietia's diffuse BRDF, and regenerated after each bounce. The random numbers in each bounce are uncorrelated (hashed integer versions of the intersecting ray positions) and I'm offsetting ray distances at the start of each bounce by [0.01f] to avoid shadow acne.
The last time I saw this bug was several months ago, and that time it was (probably) caused by shadow acne. What could it be now?