Usage of the phrase "you don't know what you don't know"

What is the correct usage of phrase "you don't know what you don't know"? Can it be used in formal conversation/writing?

When expressing frustration or exasperation, we often begin by saying something like, "If I had only known..."

• If I had known your plane was going to be two hours late, I wouldn't have rushed to the airport.
• If I had known you were going to get such bad grades on your report card, I wouldn't have let you go to the dance last weekend.
• Had I known the van was going to overheat, I would have driven the car instead.

Yet, in all these instances, life offers no crystal ball; there's no way of knowing what the future holds, or to be cognizant of something we are unaware of. This is the essence of "You don't know what you don't know," only it's being expressed in a comical way, much like Yogi Berra might have said.

(To those unfamiliar with the Yogi Berra reference, he was a professional baseball player who was reknowned for his wry way of saying things, such as, "It ain't over 'til it's over," and, "You can observe a lot by watching." That said, you couldn't know what you didn't know, but now you know.)

Generally speaking, it would not be a good way to express something in formal writing, unless perhaps you were deliberately injecting humor.

Socrates said 'I know that I know nothing', which seems to cover the sentiment. I'd say his version is worthy of formal writing or conversation.

One of several sources of this and similar phrases is a speech by US Secretary of Defense Donald Rumsfeld on February 12, 2002:

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.

Whether "unknown unknowns" can be considered "formal" or not in the strictest sense, it is, thanks to this speech, a familiar phrase, reasonably well-understood, to folks in the US who are older than maybe 30 years of age.