What was odysseus' fatal flaw?

Arrogance. The reason he invoked Poseidon's wrath was due to him revealing his name to Polyphemos. Polyphemos (a cyclops, if you don't remember) was not aware of who Odysseus was, since Odysseus had introduced himself as 'nobody'. However, his arrogance led him to reveal his name as he sailed away. Therefore, Polyphemos was able to tell Poseidon, his father, the name of the man who had wounded him. Which leads to Odysseus' famous wanderings.