We may fall into sin not knowing we are sinning, and find it out only later. When we come to see that sin we need to confess it to God and receive His forgiveness. As long as we live with this flesh it will be necessary for us to confess these sins to God and receive forgiveness (1Jn.1:9). If someone imagines he isn't sinning in any way he is walking under a delusion and resisting the voice of the Spirit (v.8). If we walk honestly before God we will see that even after becoming children of God we have to deal with deeper and subtler forms of sin every day.
But there is a big difference between falling into sin against our wishes and choosing to sin knowingly and deliberately. Paul testified from experience that he found himself doing things he actually hated because of the 'law of sin' that was working in his flesh (Ro.7:15,17). This means that he didn't like what he found himself doing, and he didn't actually want to do them. But he found that the sinful nature in his flesh was doing things which he really hated.
Because of his new birth through the grace of God and his faith in Christ (Ep.2:8,9), he had received a heart that did not want to sin (1Jn.3:8,9). But he found himself doing sinful things he hated in his heart. Anyone who is honest and sincere can identify himself with this kind of experience.
God wasn't seriously disturbed about Paul's failures because in his heart he was an honest and sincere disciple who wanted to become like Christ in all his ways (Ro.7:25;8:1). If we feel bad about ourselves when we find ourselves ending up doing sinful things even when we didn't want to, we can receive encouragement knowing that our Father knows our frail frame (Ps.103:13,14). We know also that we have an Advocate who knows exactly how we feel because He was tempted like us on all points but did not sin at all (He.4:15).
But falling into sin accidentally or unknowingly is very different from choosing to go ahead with what we already know very well to be sin. We are tempted to give in to temptation because the bait is attractive, promising us some pleasure or advantage. When we are born again, the Spirit of God tells us that what we are being tempted to do is displeasing to God (Is.30:21). But if we go ahead because we don't want to give up the pleasure or advantage, we commit sin, knowingly and deliberately. This is very serious.
In the Old Testament, there were provisions for people or even leaders to make sacrifices for doing wrong ignorantly or unintentionally (Nu.15.22-29). But anyone who sinned 'defiantly' was to be cut off from the people because he was blaspheming the Lord and despising His word (vv.30,31).
God wants to deliver us a little at a time from all sin (Ro.6:14;Ex.23:30). But knowingly committing sin is very serious before the Lord with dire consequences. If we do not stop ourselves from that, it will be unrealistic to expect that we will be set free from sin's power and become like Jesus.