Exception getting block type in world

What I expected to happen was...:
Correct world generation

What actually happened was...:
java.lang.RuntimeException: Already decorating

Steps to Reproduce:
1. Start local builtin server in creative mode
2. Fly around at a height > 255 in circles
3. Block generation and decoration will eventually fail
after several FPS drops (i.e. <1fps).

Tested on Windows 7 x64 with Java Server Edition: 1.7.0_51, an AMD FX 8350 CPU and an MSI GTX 770 GPU with updated drivers.