“But you don't know that you know that!”

Fast:

“We need to keep in mind that "I know that I know" is ambiguous between 1) I am certain and 2) I have knowledge that I know. The latter is very often true, but the former is very rarely true.”

Emil:

“Why do you think it is ambiguous between them? It seems to me that it is not ambiguous at all and only means (2).”

Fast: “Well, it may mean the second, but when someone rebuts, "But, you don't know that you know!," the implication of what they mean is almost always the first. Um, the opposite ("you are not certain") since I used the word, "don't".

The point is the utterance of "you don't know that you know" is oftentimes based on the confusion between knowledge and certainty. If you don't confuse them, then you probably don't mean the first, and even if the phrase isn't technically ambiguous, there's certainly a difference between what the person means by what he says and what the person says means.”

Source.