"Knowing what you know is fine. Recognizing what you don't know, especially what you think you do know, is a valuable skill."

- Fejj

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Older versions....

Knowing what you know is fine, but "knowing" that what you believe in is true is dangerous.

Or better like this?

Knowing what you know is fine, but "knowing" that what you believe is true really is true, is dangerous.

This was almost.

Knowing what you know is great, but recognizing what you don't know is a valuable skill.

(This is my own principle I made up. There were other versions in the past. Maybe if I wrote them down, I'll find them sometime and I will add them here....)

I will try to briefly explain. This is another thing stemming from egoism. Everybody thinks they know everything basically. But with exception of some specific knowledge which they accept a professional knows or at least better.

This is best understood at abstract level 2 or higher.