Yoma 36 Confessing for the Sins of our Fathers

The Maharsha points out that when confessing a sin which is unintentional we say that we sinned together with our fathers, however by an intentional sin, we say that we sinned but make no mention of our fathers. He explains that when we sin with intent or to rebel, that is obviously the person's own fault and nothing to do with his father, however by an unintentional sin, we can say that his father could have taught him better. It is interesting to note that the Rambam in Hilchos Teshuva does not mention that we confess the sins of out fathers, however by his nusach hatefilah, he does, in accordance with our minhag. For further discussion, look at Shearim Mitzuyanim B'halacha.