Who can discern his errors?
Acquit me of hidden faults.
The word "his" is referring to the servant (I believe). And really, who can discern or understand all of their own errors. We say so casually that "we are always sinning." But do we really believe that?
We are all sinners. We have all fallen short of God's glory.
The Psalmist believes that. This is why he prays, "Acquit me of hidden faults." He knows that there are too many to discern. He knows that he has sinned in ways that he hasn't even noticed.