Put the summary of the bug you're having here
What I expected to happen was...:
I was playing in superflat, expected in the f3 menu to show superflat or at least plains under biome
What actually happened was...:
It said I was in an Extreme Hills biome
Steps to Reproduce:
1. Make a new world in superflat
2. Open f3
3. Sometimes it might appear Extreme Hills in biome
The superflat world was generated with an extreme hills biome. In superflat, this does little more than change the color of the grass, for example, you can specify "Desert" as the biome, and the grass will be the color of grass in deserts. You can specify which biome generates in the superflat world in the world presets.