Well, I've searched for "floating blocks" and "extreme hills" bugs and haven't found this reported (unless I'm really clueless), so here I am

While trying out 14w31a, I noticed that in almost every Extreme Hills biome I found, there were some weird floating block formations, from small islands to columns made of dirt, grass blocks, stone and gravel. They weren't in any way connected to any other structure so they could simply be a weird rock formation (they were totally floating), so that's why I'm reporting them.

How to replicate: Generate a new world with the default settings, then go to several Extreme Hills biomes.

Seed of the last world I tried (has this bug): -2283811933278243749

Hope it helps!