Too bad there's not a complimentary document, "How Rsync Breaks", because that one would be quite useful as well. I've had it fail in the most annoying and arbitrary ways and it's dissuaded me from using it in any real production situations.
The way rsync works, it CANNOT end up with a partially updated file! (unless you use --inplace or --append which implies it - and it's your fault if you do)
On Windows there's Transactional NTFS whereby you can bind an open file to a transaction and then have either all or no changes applied at once. But that's only Vista+ and I am pretty sure rsync doesn't use it anyhow.
It does cost another copy of the file on disk, but it does NOT leave the file in an inconsistent state. It is either the original file, or the new file - no in between.
You CAN avoid this behavior, by using the "--inplace" switch or the "--append" switch, which tell rsync to just modify the file in-place. However, this is NOT atomic, and NOT the default (for that exact reason).