Lemon shaped black spots is a well-known Poser render bug since P6 (at least). It appears on some surfaces when using raytracing (AO can trigger this, but you get it also without AO), and it's hard to get rid of. Playing with the lights' or AO's shadow Bias, as IsaoShi said, is the usual way of getting rid of them, at a cost though, for it makes your shadows/AO look less realistic...  :-(

What I do, when I don't get too many of those black lemons, is to render a partial render without whatever triggered it (AO or raytraced shadows), and use that patch to postwork the problem.

It's something I'd love to see taken care of...