I heard that Prince William of Glücksburg, second son of the future King Christian IX of Denmark, was actually enthroned as King George I of the Hellenes before his own father was enthroned as the Danish reigning monarch. I already knew that the man we know in history as King Haakon VII of Norway was enthroned before HIS own father, the future King Frederik VIII of Denmark.

Are there any other examples in history of sons who acquired thrones before their own fathers? The example of King Michael of Romania doesn't really count, since his father (the future King Carol II) supposedly renounced the throne -- only to take it away from his son later on.

If a son became a sovereign before his father, it obviously would have to have been through the branching out of a royal house (we have discussed the Habsburgs, Bourbons, Coburgs, etc). This situation would be very unusual, and very different from a younger son reigning before an older son.