Carlos III's father, Felipe V might be considered an uninteded heir, being "just" a grand-nephew of Carlos II.
Besides the central powers were backing ADke Charles for the spanish throne to prevent the franco-spanish block.
In the end, the death of the austrian Emperor, paving the Viennese throne to ADke Charles, reverted the equation.
Another unintended heir was Franz Josef I, who would not have become Emperor, if his father had not renounced the sucession.
The last Valois make a nice example:
François II died unexpectadily aged 16, and he was followed by his next brother Charles IX who died at 24 leaving one daughter, and was succeeded by the next brother Henri III.
At his death the crown passed to his distant cousin and brother-in-law Henri IV, another unintended heir.
In Portugal, King D.João II had one only son with his wife and cousin Queen D.Leonor.
When Crown-Prince D.Afonso died rom a horsefall, the king tried to appoint as heir his illegitimate son D.Jorge but faced the fierceful opposition of the Queen.
In the end,the unintended heir was the Queen's brother D.Manuel, the 9th and only surviving male child of the Dukes of Viseu.
D.Manuel's 5th son was Cardinal D.Henrique.
He became king after the ill-fated battle where his grand-nephew King D.Sebastião died in Morocco.
He was the last survivor of D.Manuel's offspring.
Louis Philippe d'Orléans could be considered an unintended heir after the Bourbons have been sent to exile.
In Spain the most interesting case, imo, is Carlos III. As the 3rd son of Felipe V he was surely an unintended heir. If was so, that his parents secured another Kingdom for him. He became first Duke of Parma and later King of the Two Sicilies. He had to abdicate it to become King of Spain, when Luis I and Fernando VI died without an heir. I do not know of any other case, being the monarch of 3 different countries in his life.
I was assuming that the OP was looking at younger brothers who replaced thier older brother in the line of succession as the meaning of unintended heir.
But if unintended heir is anyone who replaced the heir then many more will fit that definition such as George III or Juan Carlos.
I'm fine with either scenario.
How would you classify Juan Carlos, king of Spain by grace of Franco, while his father, the CoB, the rightful heir, was still alive and was displaced ?
JC, too, was the heir to the heir who inherited earlier than expected.
I thought of that too but doesn't unintended heir mean that they were not intended to be the heir? George III was going to be intended heir it's just that his father's death made him the intended heir a bit sooner.
George III might count since the heir was his father Frederick Louis, the prince of Wales, who died before his father.