Jocasta's hamartia, or fatal flaw, is that she thinks she has beaten the gods. When she and her husband learn that it is prophesied that their newborn son is destined to kill his father and marry his mother, they decide to expose him on the side of a hill to die. That way, they don't have to worry about the prophecy.
It never occurs to Jocasta that she can't beat the gods. Like Oedipus, she lacks sufficient humility. She believes the baby is dead, but, in fact, it has been saved by the shepherd she thought had exposed it, who gave it to a couple in Corinth.
When Oedipus arrives in Thebes, just after her husband has been supposedly killed by "robbers," it doesn't occur to her that this may be her son, even though he has just killed her husband.
Related to the flaw of thinking she could defy her fate, Jocasta also tries to hide from that knowledge. As the story is unfolding and she is realizing that she has married her son, she wants to go into denial and simply not hear the truth. When she is forced to accept what has happened, she kills herself, seeking oblivion.