I'm really not sure how to describe this, but two of my upper teeth, the lateral incisors (yeah, I had to look it up so you dental people could understand :P) each have a weird extra layer of tooth-like part in the back that seems to be coming down from the gums. So when I feel those two teeth in the back with my tongue (near the gums), it feels like there's another tooth or something that's attached in the back to each of the lateral incisors. It feels like the surface on the back of those two teeth are uneven at the gum line.
Why do I have those layers or whatever they are? Sorry, it's extremely hard to describe it, and I don't blame you if you aren't able to picture it.
EDIT: Added a picture below. No need to criticize my horrible doodling :P http://i56.tinypic.com/25sbywi.png