There are many different ways a pastor may be derelict in his duty. The most common and obvious would be his morality. If a man is not reflecting the doctrine that he is teaching then his ministry is a sham. We know that there are moral qualifications for the office of elder (1 Tim. 3:1-8). At the same time the pastor must be biblical in his doctrine; he must have a firm grasp on the truth. If he is in error doctrinally then his congregation will suffer. As a result Paul gives many encouragements to this end in 2 Timothy alone (2 Tim. 1:6-7, 13-14; 2:15; 4:1-4, etc). This culminates with the pastoral inclusio to watch your life and your doctrine closely (1 Tim. 4:16).
There is another aspect where a minister of the gospel may go wrong, and I fear it is becoming increasingly neglected or at least overlooked. He must give attention to his tone. The pastor is to be firmly committed to the truth while maintaining a tone that is consistent with the truth. In other words, truth and tone go hand-in-hand. If I might take some liberty, “what God has joined together, let no man separate.”