What does it mean for someone to sin “unintentionally”?

The word unintentional may be translated as, “inadvertent sin” or “unintentional mistake.” The statement in Leviticus 5:17 that the sinner did not know about the sin indicates that these sins were not premeditated or intentional. Rather, these sins reflect an individual's ignorance or lack of awareness regarding God's law. This type of sin may be distinguished from sinning against the Lord with, “a high hand” (see Numbers 15:25, Numbers 15:30). The distinguishing mark between inadvertent sin and high-handed sin is the mental state of the person who commits the offence (see Numbers 35:11, Numbers 35:15). Nevertheless, unintentional sins are still serous and incur judgment. Currid (p.53) notes that, “Even unintentional sins need to be taken care of through sacrificial ritual—sins committed through accident or ignorance still result in condemnation and they need rectification.”