What could have been!
David Bronstein treats us to a memorable moment in Zurich, 1953. This game can be found in his highly-esteemed book on the tournament, game number 130. Here, we have two grandmasters missing a mate-in-two twice. You begin the count when Reshevsky played 20...Bf6??. As you can surmise, 21. Qg6+ leads to a mate in two. That's one. Szabo responded with 21. Bf6??. That's two. There are no hard lines of analysis here, only a simple truth. The mate-in-two stood out like a wart on a bald head, but neither player noticed. Finally, Reshevsky got the idea and played 21...Bd5 getting rid of the pin on the f7 pawn.