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. – Donald Rumsfeld
I quoted this quote before in my article Unknown Unknowns. Here we’re dealing with known knowns, some of which can appear unknown when one sits down at the board.
For instance, I knew better than 5. Bd3. When Black plays a peremptory 4… d5 in the 4. e3 line of the Nimzo Indian, Black has forfeited the opportunity to take on c3 and set up the attack on the d-pawn with c5 & e5 to neutralize White’s center or force it to advance to d5 and become immobile and vulnerable to lateral attacks, or even a kingside attack. The correct response is 5. a3!
Later, I saw 18. Rxc6!! but played the fishy 18. Bf4!? instead. It’s hard to explain. I think I may have been hoping my opponent would just drop his queen.
Finally, an unknown unknown: Stockfish finds the interesting 25. Rc5! which I certainly did not consider. It’s a waiting move which puts Black in near zugzwang: the White rook and bishop restrain Black’s two rooks and doubled f-pawn. Once either Black rook leaves the file of its partner, White’s invasion of the seventh rank has no answer.