In book One of The Odyssey, what did Poseidon have agianst Odysseus?
When Odysseus lands on the island where the cyclopes live, he encounters Polyphemus, Poseidon's son.
Polyphemus captures Odysseus and some of his best men and traps them in his cave. He then starts to eat some of the sailors. Odysseus shrewdly finds a way for all of them to escape (in part by blinding the Cyclopes) , and makes something of a fool of Polythemus in the process so that when the one-eyed giant goes to his friends for help, he sounds ridiculous and they ignore him.
Meanwhile, Odysseus and his men return to their ship, stealing livestock along the way. When Polyphemus realizes what has happened, he finds his way to the shore. Still unable to see, he starts to throw large stones at Odysseus' ship in an attempt to sink it. Odysseus, full of his own sense of importance, turns his back on the hero's code: he not only identifies himself to the giant, but taunts and makes fun of the fact that someone so much smaller in stature (himself) could outsmart the hulking creature (Polyphemus).
Polyphemus is furious by the insults Odysseus throws at him (and his inability to beat Odysseus) and so he calls on his father, Poseidon, to punish the egotistical Odysseus. Poseidon does, in fact, answer his son's request, interfering with Odysseus' journey making it that much harder, and taking that much longer, for our hero to return to his native shores.
This is the main reason why it is so hard for Odysseus to get home. (It takes him ten more years!)
But it's also interesting to note that Poseidon's fury at Odysseus is augmented by his rivalry with Athena, who is Odysseus's protector and patron goddess. Poseidon and Athena once competed over who would be the patron god of the city of Athens. Athena won, obviously, and the city was named after her. Poseidon has been a bit resentful of that since.