Creating a superflat world with a new biome and then switching to a new version without that biome causes any new chunks to crash the game.

Steps to Reproduce:
1. Create a Superflat world with the mega taiga or another new biome.
2. Load it in a version without that biome.
3. All foreign biomes will be turned into plains biomes.
3. Any new chunks trying to generate the new biomes from the unchanged presets will result in a crash.

Edit: I just thought of checking what the f3 thing says when you are in a "new" biome, and it seems that the game has turned all previous chunks into plains biomes, however, the preset tries to generate the new biomes when traveling. Interesting. I have updated and cleaned up the description accordingly.
This bug is more of a backwards compatibility failure, so I don't know what to do about the affected versions. The bug is still there, just no new biomes have come out. When/if more do come, this will present a problem to 1.7 and below, rather than just 1.6, so I will put it as affecting 1.7 because it will when new biomes come out.