Look out for the teaching and hypocrisy of the Pharisees.
Jesus was telling His followers to be careful what they listened to. On another occasion (Matthew 23:1-11) He warned them that the real problem was not the content of their teaching but the conduct of their lives. In God's eyes, those are connected. Hypocrisy, to do one thing and say another, is a serious matter. The priorities of the Pharisees were out of order. They valued appearance above substance. They valued honor from men more highly than honor from God. This corrupted their lives and therefore their teachings.
We need to be careful to examine our motives and live for an audience of One. Our lives need to match our words. We need to guard our hearts from hypocrisy.