It's hard to say when it does it.....once it was only two figures in the scene with a simple light set-up and recently it was a 5-figure scene. It doesn't matter what I set the render settings to, once it happens to a scene, it won't render no matter what I do.

I normally just re-start the scene from scratch, so I don't have any screen shots right now. I'll get one the next time it happens tho.