When importing a quad mesh with "Keep quads" enabled (in mesh importer), some faces are seemingly ignored by the progressive pathtracer. It appears rays just go "through" the faces without an intersection.

Repro steps:
1) Open attached project.
2) Toggle directional light mode from Realtime to Baked.
3) Observe artefacts on mesh and in the shadow on the ground plane.

The included test mesh works fine if you switch to Enlighten or dynamic lighting.