"Thou hast rebuked the proud that are cursed, which do err from thy commandments." (Psalm 119:21)
In "Biblical Perspective" Gordon Snider writes:
These are people whose sense of self-importance causes them to accept authority for themselves which belongs only to God. And this naturally leads to a refusal to follow the directions of God. Thus, to the psalmist, to be proud is to be cursed. The proud were specifically described as those who do err from thy commandments. The connection between the proud and do err at first seems unusual. The proud deliberately choose to resist God, while err means to sin through ignorance. There may be a suggestion here that open rebellion against God makes it impossible for us to see the “road signs” along the way that indicate we are going the wrong direction. Thus we truly are not only ignorant of our wrong, but willfully ignorant.
Source: Studies in the Psalms: Adult Teacher's Insights, page 73.