Unintentional sin

The Bible clearly says that we can sin without intending to do wrong.  While ignorance of the law is no excuse in the secular world, it seems harsh that God would judge us as sinful when we didn't intend any harm.  We look at Adam's original sin, the command God made of Moses to punish those who sin unintentionally, and the New Testament spin on sin.  Our goal is to figure out whether we can indeed sin without meaning to - and if so, how does this come about?