After spawning into a new world that I had created (using the custom preset "isle land"), none of the chunks were visible. It was the type of bug where you are found slowly sinking into an unloaded chunk in a sea of darkness, and as I got to Y=0, I was able to walk around (slowly) just above the void (see attached image). After hitting ESC and pausing the game, I returned to find my head in bedrock, at which point I fell to Y=-300, and then popped back up to the surface. This was the second world I had loaded in 14w30c, except the first time I only sunk about 10 blocks before the world loaded.