The Church and Fallibility

Imagine someone hearing of Christ and his claims to be the Son of God, and goes to listen to him. Later, a friend asks him what he thinks, and he says:

"Well, I am convinced he is truly the Son of God, but I can only accept 99% of what he ways. 1% of it I find morally repugnant."

This is essentially what people are saying when they say "I'm Catholic but I think the Church is wrong about X." The Catholic Church holds itself to be authorized by Christ Himself to speak with His voice, guided by the Holy Spirit. If you think it can be mistaken on a matter of faith and morals, then it means you either think God can be mistaken about such things, or that the Catholic Church doesn't really speak with His voice. Either way, your difference with the Church's understanding of itself is not trivial but fundamental. It doesn't matter whether you disagree with 1%, 10% or 100% of the teaching; the logic follows no matter the percentage.