http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2524489-realistic-terrain-generation-rtg-realistic-biomes?comment=3348

I think this is more of an issue with the way the surfacing is done in Mesa biomes, but worth a look either way.

I was going to try to reproduce it before opening an issue here, but oh well. :P

After examination of the the chunks, it appears that this is the x/z flipping issue (#920), so closing this.

Proof: