In The Story of Christian Theology, theologian Roger Olson writes:

A popular misconception—perhaps a Christian urban legend—is that the United States Secret Service never shows bank tellers counterfeit money when teaching them to identify it. The agents who do the training, so the legend goes, show bank tellers only examples of genuine money so that when the phony money appears before them they will know it by its difference from the real thing. The story is supposed to make the point that Christians ought to study truth and never heresy.

The first time I heard the tale as a sermon illustration I intuited its falseness. On checking with the Treasury Department's Minneapolis Secret Service agent in charge of training bank tellers to identify counterfeit money, my suspicion was confirmed. He laughed at the story and wondered aloud who would start it and who would believe it. At my request he sent me a letter confirming that the Secret Service does show examples of counterfeit money to bank tellers.

I believe it is important and valuable for Christians to know not only theological correctness (orthodoxy) but also the ideas of those judged as heretics within the church's story. One reason is that it is almost impossible to appreciate the meaning of orthodoxy without understanding the heresies that forced its development.