Chrsit warned against the Pharisees because they were guilty of saying one thing but doing something different. What they said was true but what they did was wrong.
How does this apply to preachers? Is it not because preachers are always "saying" and what they say must be true? But can they live perfectly? None have been known to do so. But he cannot have the attitude, "do as I say and not as I do." He must strive to practice in life what he preaches from the pulpit.