I was attempting to apply ambient occlusion to a model of space filling molecule and was getting artifacts crowding around the corners where the spheres intersect.
I tried playing with every imagineable setting and the artifatcs just werent going away.
The solution was to increase the subdivision of the sphere primitives (so balls are smooth)and turning off the render perfect option. (could still use some more subdivision)
I suppose AO runs into problems working out virtual geometry.
Hope this helps someone.