King Who Watched the Wrong Sail
Aegeus is king of Athens, and father to Theseus by way of a son he doesn't learn about until the boy arrives, grown, having cleared the road from Troezen of every monster on it on his own initiative. He loses him again almost immediately after: when Theseus sails to Crete as part of the tribute owed for the Minotaur, father and son agree on a white sail to signal his safe return.