Kingdom of No Sicilies

Explanation From Original OP:

The colloquially called Kingdom of Naples was actually officially called the Kingdom of Sicily.

That is because after the island was conquered by Peter III of Aragon and he was proclaimed King of Sicily, Charles I of Anjou (at the time ruler of both the island and the rest of southern Italy) retreated to the mainland, transferred the capital from Palermo to Naples, and was never able to retake the heart of his kingdom. Since the pope was the one who bestowed royal titles, in order for Charles to keep calling himself king, he was forced to maintain the kingdom’s old denomination despite having lost its namesake.