Looking at the scene, I am going to go with having two skies and using compositing tags as being the issue. Not really how skies are intended to be used, there should (like in the real world) only be one. If I delete the second sky and remove compositing tags from it, the artifacts disappear.
Not sure what effect you were going for there with the two skies? You could use a refract and reflect override for the scene (in the Scene environment tab), to place a different environment for reflections and refractions from materials in the scene, which is the intended workflow. That way your single sky is providing the lighting, but your materials will reflect and refract "something else".
