In a world generated with 14w29b, chunks from jungle and extreme hills biomes were generated as AMPLIFIED terrain, despite the world being set to Default terrain generation.
The borders between normally generated terrain and (supposedly) erroneous terrain are flat walls which form on chunk boundaries.

The world seed is in the first screenshot.

The second screenshot and after have more memory allocated to mine craft in them because the game kept freezing as I was flying around trying to get them.

I cannot reproduce the issue, as irresponsible it may sound.