In Numbers 15:22-29 the Lord tells Moses what the people should do if they unintentionally fail to keep the Lord's commands. Whether it's the whole community or just one individual, an animal is to be sacrificed to make atonement for the unintentional sin (a young bull and a male goat in the case of the whole community (v24) and a female goat in the case of an individual (v27)). In either case, it's the priest who offers this atoning sacrifice on behalf of the people who sinned unintentionally (v25 and v28). After the priest offers this atoning sacrifice, the community or the individual who sinned "will be forgiven" (v25, 28).
Have you ever broken one of God's commands unintentionally? All of us likely have. For example, you may have spoken careless words (or done something carelessly) and ended up hurting someone as a result, though you weren't trying to hurt anyone. Or you may have been rude or ungrateful toward someone without even trying to be.
Sometimes we sin against God and against people without even trying. Sin doesn't have to be intentional to be sin. That's why David prays in Psalm 19:12-13, "Who can discern his errors? Forgive my hidden faults. Keep me also from willful sins; may they not rule over me." Sometimes we sin without even trying to - our "hidden faults". Other times, we sin intentionally, or "defiantly", as Numbers 15:30 put it. Both are sin. Should we find ourselves sinning unintentionally or intentionally, in either case we need God's forgiveness.
Praise God! Just as God provided a way in Numbers 15 to forgive someone who sins unintentionally, God has provided a way for us to be forgiven when we sin unintentionally. The atoning sacrifice for all our sins -- both willful and unintentional -- is Jesus. Jesus Christ died for all of our sins -- our hidden faults as well as our willful sins -- so that we could be forgiven and so that the debt we owed God could be repaid.
So whenever you sin unintentionally, don't shrug it off as, "Well, no big deal. I didn't mean to." Rather, confess that sin and thank God that He sent Jesus Christ to forgive your unintentional sin. And if that unintentional sin hurt others, be sure to confess your sin to them and ask for their forgiveness as well.