Even Joseph Smith knew that...some revelations are from God, some are from man, and some are from the devil.
Brigham Young knew it too:
"If a man is called to be a Prophet, and the gift of prophecy is poured upon him, though he afterwards actually defies the power of God and turns away from the holy commandments, that man will continue in his gift and will prophecy lies.
He will make false prophecies, yet he will do it by the spirit of prophecy; he will feel that he is a prophet and can prophecy, but he does it by another spirit and power than that which was given him of the Lord. He uses the gift as much as you and I use ours."