You might be suprised to realize that this is more common than you think.
Historically, Protestants have been taught that Roman Catholicism is a perversion of Christianity -- that it is the result of an unholy union between Christianity and paganism. Hislop's The Two Babylons still looms large in Protestantism. They've been taught that Catholicism is Mystery Babylon, Mother of Harlots, Queen of the abominations of the earth. (Thats Rev. 17 language, BTW).
In recent years, though, the pendulum has begun to swing the other way. Due in large part to the parachurch organization movement -- PromiseKeepers and Campus Crusade, for instance, and also the pro-life movement, Protestants and Catholics are getting to meet each other in religious settings and finding out, hey, this guy's not so different from me. A whole another world of Christians is opening up to Protestants.
The pendulum may have swung too far the other way: now, there's a tendancy amongst Christians (evangelical Protestants, anyway) to minimize the doctrinal disagreements between evangelicalism and Catholicism. There are some very real and very serious theological disagreements that we cannot simply pretend do not exist. Veneration of Mary is a good example. To an evangelical Protestant, the veneration of Mary violates the very basis of monotheism. To a Catholic, disagreement with the veneration of Mary is an attack that evokes the same emotions when someone insults one's own mother.