Superflat world with a piece of normal world in it.

I tried to create a new superflat world to test out some redstone creations. Instead it generated a piece of normal world in the superflat world.

What I expected to happen was...:
To get a normal superflat world.

What actually happened was...:
It generated a piece of normal world in the superflat world.

Steps to Reproduce:
There isn't really a way to reproduce it, it only has happened twice now.