I've never seen this before. What does the code look like?
The render is written in Haskell. I'm not sure if many people here are too familiar with it but here's an intersection routine for spheres: https://gist.github.com/4078346
I've seen something very similar to this this, turned out to be precision issues with shadow rays, that is is the ray was intersecting with the surface it was shot from, even though it should be starting from the surface, adding a small offset/bias fixed the problem. it might be a similar issue in your case.
I suspected floating point precision and already have a minimum distance I will allow for intersections (epsilon in the sample above). I've tried increasing it with no effect. How similar was your example to this one?