MAJOR chunk error

Put the summary of the bug you're having here

What I expected to happen was...:
A Superflat world with lots of biomes.

What actually happened was...:
A very odd error in world generation.

Steps to Reproduce:
(I don't know if this caused the bug, but I was doing it before loading the world)
1.go to Superflat Customize and go to Presets
2. in Preset text box I'm pretty sure I typed something like biome_1,biome_2,biome_3,biome_4 or something in it along with the default typed things but also making the world 64 blocks high with dirt
3. ...