No, the viewport wasn't paused when I tested it; that's only how the file was saved. I can't render the scene on my GPU since it uses too much memory for my card. It'd be great if someone could test this on a GPU with more VRAM.

1. Same result; in fact it doesn't matter how many threads I use.
2. Tried 'Top to Bottom' - same result. It appears to get stuck on the same tile as when I use 'Center'.
3. In the viewport it gets stuck rendering the 6th sample and can't be stopped. If I make the viewport smaller, the number of samples I can render without freezing increases from 6 to about 10 or so.
4. This shouldn't be relevant - but I am using an NVIDIA GTX 460 on my 3770K system and the integrated GPU (Intel Iris 5100) on the 4288U.

Forever - the last tile never finishes rendering and uses 100% CPU for the thread rendering the tile.

On one machine I tested I was using an Intel i7 3770K and on another an Intel i5 4288U.

Appears to be related to volumetrics, as disabling them causes the render to complete successfully. I've also tested this on OS X and Windows, so it doesn't appear to be a platform-specific issue.