The Greek words used by Paul suggest that sin in an unplanned and unexpected way is in view.1 It could be a manner of thinking or a lifestyle habit that the believer does not realize is actually sinful and wicked.2 Jesus gave instructions of how to respond when a fellow believer sins (Matthew 18:1–20). It is important to remember that restoration, and not punishment, is always the goal.3
1 Brothers, if anyone is caught in any transgression, you who are spiritual should restore him in a spirit of gentleness. Keep watch on yourself, lest you too be tempted.