Third nipple piercings, which include some great shots of very functional-looking supernumery nipples (and some that look like little mole/freckles:
http://www.bmezine.com.nyud.net:8080.../third001.html
I lack any evidence of any sort of third nipple, but I have a few spots on my breasts, an inch or more away from the edge of my areola, that will occasionally leak a tiny droplet of milk when I'm very full. Without the droplet, the skin looks perfectly like the surrounding area, just tiny pores. But then, BAM, milk.