When using the world customization, if you use a preset code that contains a fixedBiome with a value of 8 or greater, Minecraft will add 2 to that number upon clicking Use Preset. Another way to invoke this bug is, from within the world customization menu, select a biome of FrozenOcean or greater, click Presets then Use Preset, and see that the biome has advanced by 2 even though the preset code was not altered. If you try this with the last two biomes (Mesa Plateau F or Mesa Plateau), Minecraft will crash.