Deuteronomy 18:20-22 in the New World Translation Bible of Jehovah’s Witnesses warns:
“However, the prophet who presumes to speak in my name a word that I have not commanded him to speak or who speaks in the name of other gods, that prophet must die. And in case you should say in your heart: ‘How shall we know the word that Jehovah has not spoken?’ when the prophet speaks in the name of Jehovah and the word does not occur or come true, that is the word that Jehovah did not speak. With presumptuousness the prophet spoke it. You must not get frightened at him.’ ”—Deuteronomy 18:20-22, New World Translation
This Downloadable PDF packet provides photocopies from official Watchtower literature documenting the history of Watchtower prophetic failure. The simple 20 questions given along with the documentation photocopies in the packet will help you present this information to the next Jehovah’s Witness who comes knowing at your door.