What I expected to happen was...:
terrain to generate as normal

What actually happened was...:
on one world trees were cut in half vertically, and in another there were random chunks of ocean biome water inside forests and deserts

Steps to Reproduce:
1. create a world (i was in creative with cheats on and large biomes. have not tried regular biomes. superflat with a biome attached to the custom data seems ok.)
2. generate some terrain
3. most likely will find very conspicuous anomalies with the landscape

done in creative with cheats, large biomes, random seeds (seeds are in screenshots), and single player, no lan. superflat seems normal and have not tried with regular world generator.