According to Wikipedia:
According to contemporary rumors – probably current as early as 1829 – Kaspar Hauser was the hereditary prince of Baden who was born on September 29, 1812, and who, according to known history, had died on October 16, 1812. It was alleged that this prince had been switched with a dying baby, and had subsequently surfaced 16 years later as Kaspar Hauser in Nuremberg.