Errors and mistakes are two different things. As God's offspring, we are prone to occasionally making mistakes due to our imperfections. People who are willing to obey God with all of their hearts are what God seeks and desires, not flawless individuals.
In certain cases, our blunders will be readily apparent to all. Although David was not flawless, the fact that he had received some instruction set him apart from the soul. His time spent tending to his father's sheep in the field wasn't in vain. He was chosen by God to be a shepherd over the flock because he knew how to provide for the needs of the people and would eventually lead them.
Saul, however, did not receive any training.We need to be disciplined if we are to genuinely carry out God's will and obey his commands. Without discipline, nothing important can be accomplished.