(I John 5:16) Are there some sins which one can commit and not incur the death penalty?

1. The wages of sin are death (Rom. 6:23). There are no exceptions! God does not categorize sin, but Hebrews 10:26-27 shows that it is a willful sin which brings “fiery indignation” - eternal death. Any other sin can be forgiven if it is repented of - if it is “confessed” (I John 1:9).
    
2. A sin “not unto death,” then, is a sin for which forgiveness is available. It is a sin committed by someone who has not willfully turned from God. A sin “not unto death” might be the result of ignorance or weakness. The sin itself carries a death penalty, but it can be forgiven if the person repents.
    
3. If any of us sees a brother who is striving to obey God and walk with God “sin a sin which is not unto death” - sin out of ignorance or weakness - we ought to ask God to help the brother recognize his error and repent of it. By doing so we will “give him life” - God will hear our prayers for him. This is the kind of concern we ought to have for one another. This is one way that we can “bear” one another’s burdens (Gal. 6:2).
    
4. On the other hand, if the brother has sinned “unto death” - if he has willfully and viciously turned from God’s way and rejected God completely, we should not pray for him because God will not answer our prayer for him. This, by the way, would not include all disfellowshipped people - ONLY those where it is determined that they have maliciously and totally rejected God’s Way so that they could not be brought again to repentance (Heb. 6:4-6).