Tao te Ching: 27

A good traveler leaves no track or trace.
A good speech leaves no flaws.
A good reckoner uses no counters.
A well-shut door needs no bolts, and yet it cannot be opened.
A well-tied knot needs no rope and yet none can untie it.
Therefore the sage is always good in saving men and
consequently no man is rejected.
He is always good in saving things and consequently nothing
is rejected.
This is called following the light (of Nature).
Therefore the good man is the teacher of the bad,
And the bad is the material from which the good may learn.
He who does not value the teacher,
Or greatly care for the material,
Is greatly deluded although he may be learned.
Such is the essential mystery.

The word for following is hsi, which can mean many other things. For instance, it could also mean to cover, pierce, or practice. The rendering used here means to follow, but there is a nice homophonic key here which rings back to chapter 2.

When you are good at something, truly good at something, you can be good at it without leaving any evidence that it was ever there. This isn't simply leaving no evidence, but a much deeper impermanence. It is something of quality that does not exist to stand out or be remembered. We judge a lot of our successes by how we will be remembered or what our peers thought of us. In fact, some judge their entire worth on this. This sort of striving does not allow for hsü. The need to create such a feat speaks of the utmost planning and drive on the part of the actor.

But the sage wouldn't reject any man or material for these flaws. If he did he would be undermining the taiji, also known as yin and yang. It would be undermining cause and effect. It is only by names such as "good" that we even know the "bad". Lao Tzu is very clear about the relationship between the two.