Prov 30:5-6 God's words

⁵ “Every word of God is flawless;
    he is a shield to those who take refuge in him.
⁶ Do not add to his words,
    or he will rebuke you and prove you a liar.

After giving these verses some good study time, I think that our guide, Agur, is trying to tell us that we cannot begin to improve on God's Word. And, additionally, he tells us not to even presume to think that we can add anything to His Words. The implication here is that God sort of takes that personally.

Of course, that has never stopped people throughout the ages from doing it anyway, has it?
Jesus condemned the Pharisees in Matthew 15:9 for "teaching for doctrines the commandments of men."
And Paul told the Christians in Galatia that "...some people are throwing you into confusion and are trying to pervert the gospel of Christ." (Galatians 1:7)

Also, Paul warned the Elders at Ephesus to be wary because “Even from your own number men will arise and distort the truth in order to draw away disciples after them.” (Acts 20:30)

Ah, so there is the key to the problem. That is what Agur was cautioning us about:  People will distort Scriptures, and add to God's Word because they want other people to pay attention to them.
They want people to follow their teachings, their new (or re-packaged) doctrines.

And that’s an easy temptation to give in to. Lots of churches have fallen prey to that. Lots of church leaders have been enticed by that.
It's a problem that has been around for a long time, and it will be around much longer. It is even something noted in the prophetic book of Revelation:

I give fair warning to all who hear the words of the prophecy of this book: If you add to the words of this prophecy, God will add to your life the disasters written in this book; if you subtract from the words of the book of this prophecy, God will subtract your part from the Tree of Life and the Holy City that are written in this book. (Revelation 22:18-19)

How can we avoid "adding to" or "taking away" from God's Word?
Galatians sums it up when it says “… the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law.” Galatians 5:22-23
If we reflect those attitudes, we’ll never become legalistic or pharisaical.
And if we strive to build our thinking around the Word of God we’ll always have God’s wisdom to guide us in our lives.