A common confusion I run into. Not that it makes the doctrine of the Immaculate Conception entirely palatable:
Another misunderstanding is that by her immaculate conception, Mary did not need a saviour. On the contrary, when defining the dogma in Ineffabilis Deus, Pope Pius IX affirmed that Mary was redeemed in a manner more sublime. He stated that Mary, rather than being cleansed after sin, was completely prevented from contracting Original Sin in view of the foreseen merits of Jesus Christ, the Saviour of the human race. In Luke 1:47, Mary proclaims: "My spirit has rejoiced in God my Saviour." This is referred to as Mary's pre-redemption by Christ.
From, of course, Wikipedia.
