Here's a render with one black dot showing up. The only illumination in the scene is a large emitter plane made of one poly. There is no SSS but there is a thin sheet of high grade glass (on the top face). The black dot showed up after I added the glass piece. I haven't re rendered the same scene again yet to see if it reappears or multiplies.

Brian Looney
Maxwell + RealFlow
http://www.turbosquid.com/Search/Artists/Bubbaloo
Help! I'm dispersing into a haze of probability!