Inconsistent Bucket Density

[Streetwise]

I used to get this on occasion with CPU renders, but on this one scene, it was rendering just fine with both GPU's, but all of the sudden, it has started doing the same thing! I thought that maybe one of the cards wasn't working properly, but then unselected the GUI card and I'm getting the same thing. Any ideas on how to start diagnosing the problem? I've attached a file with some added contrast so you can clearly see what I'm talking about. The actual render is pretty faint, but still there.

Thanks.

[matanov]

Is the background a geometry with Cast Shadows = off ?

[Streetwise]

No. It's just a plane. I think I figured it out. I reset the render engine and that seemed to have fixed it. So odd!