today was looking some models and found constant artifacts (always the same) on my GTX 660 TI when HDRI background was present.
In fear of my GPU, I checked on my laptop if this could be only a sketchfab issue, and found another class of artifact, always changing from model to model, but artifact in the end. It wasnt always present when HDRI BG was turned on, though.
The issue on my GTX660 seems to be constant. When there is an HDRI visible, it will always display the same pattern on the background. On my laptop, some models with HDRI-BG-ON seemed to be fine, but probably there could be issues with shadows like on the Viktor model that I didnt notice.