1Kings 11:41, 14:19, 29, 15:23, 16:14). I doubt if any of these ancient writers knew that their words would be chosen to be part of the Word of God.
And it doesn't help to ask at what point these documents were given "without error."
I think of God as the Historian who has brought the Bible into being through many writers and over a period of at least a thousand years. If God is not the ultimate Historian there is no point in talking about the Word of God.
But if a historian does a good job the history will be both verbal (in words that can be understood) and readable. It will also be effective in putting over a model of what is significant from the historian's point of view. And it should contain no irrelevant or misleading material.
I assume that since God was the ultimate historian of the sixty-six canonical books of the Bible, he did a good job with his history. That means I expect the Bible to be reliable and effective in putting over God's point of view. The Bible is therefore infallible in the sense that the Bible does not fail to achieve the object God, the Historian, had in mind. I certainly find that as I read it every day, it infallibly works to touch and change my life.
The word "plenary" is also used in statements of faith. I understand the world "plenary" to mean that all parts of the sixty-six books of the Bible might at some time or another become important for me, even the parts which I do not yet understand. In discussions with other Christians we find that some of them have different explanatory models for key words such as Advent, Atonement, Baptism, Incarnation, Faith, Gifts of the Spirit, Membership, Ministry, New Birth, etc.
If we believe in the plenary inspiration of the Bible we believe that any part of the Bible could be relevant to our discussion. We agree to learn from the Word of God, and engage with our differing interpretations by comparing Scripture with Scripture. We are not to be stuck in one tradition by saying "This is the proper way to interpret these texts" and I do not want to hear any other viewpoint.