Maria Amalia of Saxony (1724-1760) married the future Carlos III of Spain in 1738. Since her husband was King of Naples and Sicily at the time of their marriage she immediately had the higher title of Queen of Naples and Sicily. When her husband became the King of Spain in 1759 (and Naples and Sicily was given to their third son Ferdinand) she also became a Queen Consort of Spain. However, while she was Queen Consort of Naples and Sicily she was technically an Infanta of Spain because her husband, although the King of Naples and Sicily, was also an Infante of Spain before becoming the King of Spain. Carlos III was never Prince of Asturias so María Amalia couldn’t claim or use that title, but I don’t think she ever used the title Infanta of Spain.
I believe that Queen Amalia of Greece (born a duchess of Oldenburg) was technically a princess of Bavaria by marriage, since her husband was born in 1815 as Prince Otto, second son of the future King Ludwig I. Similarly, Queen Olga of the Hellenes (born a grand duchess of Russia) would technically have been a princess of Denmark, since her husband (King George I) was Prince William there -- second son of King Christian IX.
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?