What was Theseus's flaw?

Answer 1

Theseus' flaw was that he was a liar. Whenever he said the words "I promise" it was always broken. He promised to marry Ariadne, however he abandoned her on an island on his way home. And he promised his father he would change the colour of his sails should he return home alive, but failed to do so and so caused the suicide of his father.

Answer 2

he was a liar