Why was Poseidon mad with thesus and what did thesus forget?

Answer 1

Poseidon was angry with Theseus because, during his journey back to Athens after defeating the Minotaur, Theseus forgot to change the sails of his ship from black to white. He had promised his father, King Aegeus, that if he succeeded, he would raise white sails to signal his victory. When Aegeus saw the black sails, he believed Theseus had died and, in his despair, threw himself into the sea, leading to Poseidon's wrath against Theseus.