

Though there is a command in the law not to heed to one who prophecies 
falsely, it is still possible for the one who has prophecied falsely
to prophecy truely again.  Take, for example the story in Kings about the
man of God from Judah who came to israel and prophecied against a king.
The Lord had commanded him to not eat or drink till he returned home.
Another prophet wanted this man of God to stay in his house, so he
prophecied falsely that the Lord wanted the man of God to stay in his 
house.  While they ate and drank in his house, the Lord gave the prophet
who lied a word that the man of God would die from breaking the word of
the Lord.  It came to pass.