1 Timothy 5:20

Them that sin reprove in the sight of all, that the rest also may be in fear.

This is a special instance and does not nullify the instructions of Christ in Matthew 18:15ff (see discussion of this in my Commentary on Matthew, pp. 279-282). In the case of an elder, or other church leader, who is fairly convicted of gross wrongdoing, he should be rebuked and denounced publicly. The wise words of Lipscomb on this are:

When we cover up sins in the church, we corrupt the morality and virtue of the church and destroy its efficacy to honor God or to save men. Evil teachers and evil men must be exposed and purged out of the church, or the church becomes corrupt and becomes a synagogue of Satan instead of a church of Jesus Christ.[25]