Make a world with large biomes. When crossing over biomes the biomes don't mix together. The biomes are set at different heights.
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I couldn't reproduce this. This is probably due to some chunks in your world having been generated in a different version of the game.
I created in 13w37a.
I also couldn't reproduce this. Sometimes problems like this can also occur if you create a world, delete it and then create a new one with the same name.
Oh well that explains it.