Mine was an automated file transfer system that had to be 100% reliable on an insanely unreliable network (~95% uptime). Took about 9 months of bug squashing after development was done. So many edge cases. I would probably never mention this in a job interview because I doubt most people would understand why it was so hard.
I once wrote an inliner. When you have not done it, it seems simple. When you are doing it it is like trying to restrain a large rabid dog with a slippery leash.
Now, I am not a programmer by trade, but I have a hard time thinking anyone would find it nice to write an inliner. At least not if you want the inliner to always make things faster.
I'm working on this exact same thing. Was your code ever published or did you blog about it?
Did you ever blog about this program? It sounds very interesting, and there is no job interview on HN!