Whilst in superflat preset 'Overworld', travelled not so far from spawn (exact coordinates below) and came to a huge mass of stone and ore, like the farlands from beta 1.7, but this time on top of this mass was grass and trees. A little further on, there was normal, default generation - a birch forest and then birch forest hills, some extreme hills, taiga and a huge ocean. It was far from flat. Physics were normal. This bug means I can't really release my custom map, which was what I was building (at y 127) at the time I discovered the bug. I had decoration (grass, etc.) on, plus villages on, but no trees.

What I expected to happen was the whole world would be flat.

What actually happened was at 144, 64, 48 the generation went buggy and a default world appeared.

I have attached pictures of the odd generation.