A student of Christian Science may sometimes identify a sin or disease as "error" while simultaneously accepting it as existing and therefore valid. The result is that he finds himself believing in "true error."
Say, for example, one hears a neighbor expressing fear concerning a popular contagious disease. If the student's mental response is a bland, "Oh, that's just error," he may be accepting some part of the disease belief as true. Perhaps he sees that error—which is nothing more than a belief in God's absence or impotence—is untrue for himself, but he accepts the contagious disease as a valid part of his neighbor's life.
Simply labeling an illness "error" does not always mean that we have grasped its total lack of truth or unreality. Identifying a godless thought, statement, or action as error is an important first step. It is, however, only the beginning.