In Greek mythology, what did Theseus forget to change after leaving Crete?

In Greek mythology, Theseus forgot to change the ship’s black sails after leaving Crete.

Before Theseus sailed to confront the Minotaur, his father Aegeus was told that the returning ship would use white sails if the expedition had succeeded. The ship instead retained its black sails, either because Theseus forgot the signal or because the crew failed to change them. Seeing the black sails from the coast, Aegeus believed his son had died.

Aegeus then threw himself into the sea, which mythology explains as the origin of the name Aegean Sea. Theseus had actually survived and was returning after killing the Minotaur. This episode is separate from the thread that helped him navigate the Labyrinth, but both details belong to the same Cretan adventure.