“rebukes a mocker.” The verse does not say not to rebuke a mocker, because sometimes that is necessary. However, it makes the point that there will always be ramifications. For example, there will always be people who think the mocker was correct and the person who rebuked him was wrong, or the rebuke was too harsh, or done at the wrong place or time, or something. There are always people who criticize and “shame” those who fight for godliness. God’s people must make up their minds that doing right is not a popularity contest; we only need to be popular with God, the world will always be against godliness. Sadly, too many people who know to do what is right don’t do it because of the conflict and shame that go with it, but the world won’t be handing out rewards on the Day of Judgment, God will, so the wise person does the will of God even if it means being shamed by some people here on earth.
“brings injury.” The Hebrew is literally “a defect, a blemish,” but it comes as an injury, although the meaning also includes a moral blemish, in that case equivalent to “shame” in the first stanza, so we could almost expand it to “shame and injury.” Whereas the mocker is most likely to smear and shame anyone who tries to rebuke him, the wicked person is more dangerous and is likely to try to harm anyone who tries to reprove him. That does not mean we should not try to correct wicked people; sometimes it is necessary. But it does mean we must be aware that there is always a risk to ourselves. It is because of the shame and even potential danger of rebuking mockers and wicked people that believers have to be wise, pray for God’s help, and walk by the spirit of God.