Is it possible that a subsequent sync or subsequent write on that extended portion of the file will return an out of disk space error?
I think you're looking for an analytic answer based on theory and reasoning, rather than a practical answer based on experiment, but just in case I'm wrong: there's a rather nifty tool out there called Holodeck (http://www.se.fit.edu/boneyard/holodeck/) which allows you to precisely trigger detailed failures under your control, so that you can test exactly the scenario you're concerned about.
For example, you can cause your application to run out of disk space, or get a "too many files" error, or get a disk read or write error, at just the point that you desire. One downside is that the tool is Windows-only. I should also admit that I haven't used the tool myself, just read about it. Hope this helps, bryan
