Simple modelling of a sphere-crenellated shadows

Hi all. I’m a newbie, and I’m trying a few tentative steps on modelling with materials.
However I’ve noticed that often I get completely black shadows (on the object) but with a serrated edge. turning subsurf up a level makes the ‘teeth’ smaller, but they’re still there. Also got them when I used the monkey as a mesh.
I’ve fiddled around with the shader/material/ world settings, added extra lamps, added extra objects etc etc but I can’t work out what the problem is. Don’t get it all the time it must be said, so I don’t know if it’s an effect which is supposed to happen or not?
Help appreciated!

does it occur on smoothed objects?

try ctrl+n on all faces, recalculate normals outside

a screenshot would help

I’m not sure How I did it-I’ve been trying to recreate a screen shot, but I haven’t repeated the problem since!
So now I don’t know what the problem is, if it’s with my computer or my blender skills.