Some Pharisees . . . asked, "What? Are we blind, too?"
Jesus said, "If you were blind, you would not be guilty of sin; but now that you claim to see, your guilt remains." -- John 9
A good friend in junior high attended an Apostolic Pentacostal Church for a while where not only were they not allowed to dance, watch movies, or listen to rock music, but the women were not allowed to wear pants, cut their hair or wear makeup. Her mom thought she was in a cult and forbid her to go, and she started going to church with me and ultimately cut ties with the other church and its beliefs.
We had some interesting conversations, though, about that church and its people. If a woman in this congregation genuinely believed that cutting her hair was sin, an act of rebellion against God, and she cut her hair . . . did she sin? We concluded that she did -- she was choosing wrong over right, rebellion over trust. It was a matter of the condition of her heart toward God.
Of course, this then begs the question . . . if someone has a right heart toward God and commits a sin not knowing that it is a sin, did they sin? The passage I quoted earlier from John 9 seems to imply not. I remember a woman in Bible study in NJ very innocently telling us all how couples really do need to have sex before they get married to be sure they are "physically compatable". A new believer, and quite apparently ignorant that the traditional Christian stance against sex outside of marriage was not a matter of tradition but a command from God in scripture. (An easy mistake, actually, for one who doesn't know the Bible well -- there are many church traditions that have come to have the weight of scriptural mandate in some communities.)
Then, of course, this begs the question . . . why tell people that they're sinning if, in their ignorance, they are also innocent? A valid question, and a tough one to answer in the abstract. I actually do think there are situations where we are probably better off keeping our mouth shut and allowing the Holy Spirit himself to convict a person of their sin. As Pharisaic as my tendencies are, I would rather trust the Spirit to do his job there than overstep my bounds and condemn another of a "sin" that I'm mistaken about.
But the fact about most sins in the Bible is, the reason God tells us not to do these things is because he knows how they will hurt us. He made us and really does have the right to tell us how we should live our lives if we want to be happy. He knows that saving sex for marriage makes the sex thing so much better and our marriages so much stronger -- and he knows the "physical compatability" thing all works out in the end.
So, confronting someone regarding sin in their life (something we are instructed to do with fellow believers) is a delicate thing. Something to be done with humility, love, discernment . . . with the right condition of the heart. And once again, the condition of our heart will determine whether our confrontation is an act of sin or an act of compassion.