Shadow acne
The speckled, moiré-patterned blotches that appear when a surface wrongly shadows itself, because the shadow map's depth resolution is too coarse for the point being tested.
In shadow mapping each texel of the depth map covers a small patch of the scene, yet the comparison looks up a single depth value for that whole patch. When geometry slopes away from the light, parts of one flat surface fall both in front of and behind the stored value, so roughly every other pixel is judged to be in shadow. The result is a jagged, shimmering stipple across surfaces that are actually fully lit.
The standard fix is a depth bias that nudges the stored depth slightly further from the light before the test — a constant part plus a slope-scaled part, since steep angles need more. An alternative is normal offset, which moves the lookup point along the surface normal instead. The trap is that too much bias trades one artifact for another: the shadow detaches from the object's base, giving peter-panning. Balancing the two is the craft of shadow rendering.