I was placing quartz down in some Extreme Hills, and I kept noticing that the grass was being replaced with water and air when i caused a block update. I took some screenshots, quit and logged back in. When I got back, I discovered I had been building on top of a lake that hadn't generated, but was still there, like some weird block 36 stuff.

What I expected to happen was...:
That I would be replacing grass with quartz, and nothing weird would happen.

What actually happened was...:
I triggered block updates of a lake that hadn't generated.

Steps to Reproduce:
1. Go to an Extreme Hills biome (I haven't tried with other biomes)
2. Break blocks, place blocks, just cause BUDs.
3. Try to find a lake.