Making errors and mistakes are not the same thing. As children of God we are prone to make mistakes once in a while because we are not perfect. God does not look for perfect people, what he looks for and desire are people who are willing to obey Him with the whole of their hearts.
There are some instances that if we make mistake the mistakes will be very obvious for everybody to see. David was not perfect, but the difference between Him and soul was that David had undergone some series of training.
His days in the field while he looked over his father's sheep wasn't a waste. God made him a shepherd over the flock because one day he was going to lead the people and he knew how to tend to them and all that they needed. But Saul on the other hand did not undergo any training.
If we must truly do God's will and keep his commandments we must be disciplined. Nothing worthwhile is achieved without being disciplined.
