Many Protestants think that Catholics must believe that Purgatory is a place, and that we believe it is some "third" place between Heaven and Hell, when in fact it would more rightly be considered only a "waiting room" on the way to Heaven, and can be thought of as more of a process than a place, as Cardinal Ratzinger has opined.
They also get the false impression that we worship Mary and/or the Saints. When the idea of intercessory prayer through Mary and the Saints is properly explained to them, while they may still disagree with the practice, they at least understand that it is not worship.
That is, of course, the basis for all Protests against Catholicism.