Did Luther have any disagreements with the teachings of Augustine of Hippo? The Wikipedia page on Luther doesn't mention any, and from my limited knowledge of Luther's books, I can't recall any either. However, there might have been some.
Luther and Augustine seem to have disagreed on:
Justification (see below)
Luther believed Augustine to be, like himself and all people, prone to err. The only area where he acknowledged disagreement between himself and Augustine is on the nature of justification:
It was Augustine’s view that the law, fulfilled by the powers of reason, does not justify, even as works of the moral law do not justify the heathen, but that if the Holy Spirit assists, the works of the law do justify. The question is not whether the law or the works of reason justify, but whether the law, kept with the Spirit’s help, justifies.
I reply by saying No. Even if in the power of the Holy Spirit a man were to keep the law completely, he ought nevertheless to pray for divine mercy, for God has ordained that man should be saved not by the law but by Christ. Works never give us a peaceful heart. Christ would never have been sad in spirit unless he had been pressed hard by the law, to which he subjected himself for our sake.
Luther may here have in mind Augustine's The Spirit and the Letter.
For more on the history of the doctrine of justification, see Alister McGrath's Iustitia Dei. McGrath contends that Augustine agreed with Catholicism that "to be justified is to be made just" and that the idea of "imputed righteousness" originated with Luther.
Luther had a very high opinion of St. Augustine. He often spoke about him in his table talks, praised him even:
"Augustine was the ablest and purest of all the doctors..."
—Luther, Table Talk DXXXI.
Reading Luther's table talks you get the sense that Luther admired Augustine, as Augustine seemed to have almost as much cynicism as Luther did towards the church:
"We must read the Fathers cautiously, and lay them in the gold balance, for they often stumbled and went astray, and mingled in their books many monkish things. Augustine had more work and labor, to wind himself out of the Father’s writings, then he had with the heretics." —Luther, Table talk DXXIX.
So unless I'm proven wrong, I'd say it safer to assume that Luther didn't have any disagreements with Augustine, and actually viewed Augustine as one of the few fathers that didn't deviate—at least not very far—from the truth. I'd go as far as to say that the church, as it was with Augustine, was near to what Luther wanted to reform the church back into.