OK. I checked a bunch of other profiles, and they all exhibit the same behavior. Must be quite recent, because I never noticed this message before (Either that, or my eyeglass prescription needs adjusting...) However, on one of the profiles I checked, I had a "== Note ==" section, and it also complains about a wrong level heading. Are there specific section names one has to have (aside from a Biography and Sources section) or others that cannot be used (such as "Resource note")?