But to the best of my knowledge, neither woman ever used the princess title, much less with the qualification of Royal Highness, since each was elevated at marriage to the highest title and qualifying rank -- HM The Queen (notwithstanding the fact that HRH Princess is a higher qualifier than HH Duchess). The same could be said of subsequent queens consort of Greece, in the house of Glücksburg.
Nevertheless, one can't argue with the facts: despite their status as kings of Greece (whether enthroned or dethroned), George I, Constantine I, George II, Alexander, Paul, and Constantine II have all also been princes of Denmark (as male-line descendants of King Christian IX). Accordingly, their wives would have been princesses of Denmark as well (with the exception of Aspasia Manos, morganatic wife of King Alexander of the Hellenes). In fact, it has been stressed, time and again, that Queen Anne-Marie today is a princess of Denmark by both birth and marriage.
Are these examples, then, but among the cases of unused titles in European royal history? That is: one might be A princess of country X, but never called that. Consider, as just another example, Sophia Dorothea of Hanover (born in 1687), consort of King Friedrich Wilhelm I of Prussia (her first cousin, son of her aunt and namesake, Queen Sophia Charlotte). Despite the accession in 1714 of her father (the elector of Hanover) as King George I of Great Britain (thanks to the Act of Settlement, which placed the British crown upon the person and descendants of her namesake grandmother, the Electress Sophia of Hanover), she was never styled as a princess of the United Kingdom, even though that's what she technically was. After all, she was already a married woman -- and a queen at that -- by the time of the said accession.
What are some other notable examples of unused titles?