Odysseus's great flaw, revealed in his adventure with the Cyclops, is a mix of recklessness and pride. Indeed, you might note that it was only due to his recklessness that Odysseus and his men were captured by Polyphemus to begin with. It was Odysseus who insisted on remaining in the cave, intent on receiving a gift from its owner, even as his terrified compatriots pleaded with him to depart. This decision would see them trapped in the cave, requiring Odysseus's cunning to ensure their escape.
However, as they are making their escape, Odysseus's recklessness manifests again. While trapped in the cave, he concealed his identity from the giant, but now that he has defeated Polyphemus, Odysseus is overtaken with a desire to ensure that the giant know who bested him. Thus, he tells the giant his name.
This proves a costly mistake, given that Polyphemus is the son of Poseidon, the Greek god of the sea. As Odysseus is sailing away, the Cyclops makes a plea to his father to avenge him. Thus, Odysseus, in his reckless pride, has made a dangerous enemy in Poseidon, who takes a personal stake in Odysseus's journey, seeking retribution for his son. Odysseus suffers greatly for his moment of pride, and while he does eventually complete the return journey home, it is as the only survivor among his compatriots who sailed with him to Ithaca after leaving Troy.