I wonder what the chances of the guy actually asking for the Machiavelli tract are relative to the chances of him being wrong about the author? When I run into a namespace collision like that, I try to be extremely clear about it precisely so that I don't run into situations like you described -- i.e. "Machiavelli's Art of War, not the one by Sun Tzu".
You're absolutely right. Anyone who knew about the existence of both books would also be aware of the need to clarify which he meant (unless he was deliberately testing me so he could feel smug at his superior knowledge). The chances he was simply mistaken are still pretty good.
Had I considered that possibility, and rejected it on grounds of low prior, maybe I would have been entitled to a Rationality Cookie; but alas, what actually happened was that I didn't think at all.
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.