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.
Linked issues
Comments 6
I fixed it but I think its important to note it does it in all builds that contain the customized world presets option.
Duplicate of MC-54484 (add the summary from this ticket there)
Please don't put down the affected version as a future version. You DON'T have access to it yet!