We can always trust the living prophets. Their teachings reflect the will of the Lord, who declared: "What I the Lord have spoken, I have spoken, and I excuse not myself; and though the heavens and the earth pass away, my word shall not pass away, but shall all be fulfilled, whether by mine own voice or by the voice of my servants, it is the same" (D&C 1:38).
Since their TEACHING reflects the will of the 'Lord', then too, those same TEACHINGS will reflect church doctrine and not contradict it - since it reflects the 'lords' will. Thus any teaching disregarded by mormons are disregarding the will of the 'lord'. Apostle's teachings are included in this as well (see D&C 1:14-16)
If they TAUGHT 'unofficial' doctrine - then they are FALSE TEACHERS and accordingly FALSE PROPHETS iaw mormon doctrine above - pure and simple.
My nomination for the "Best FREEPER 2010 Lds post on "The Mormon 'prophet' will never lead you astray' thread" is: Kimball on Boy Scouts, Vernon Romney and Ezra Taft Benson