Resignation Trap: Dragoslav Andric's account in Chess Life & Review:
"With two extra pawns in a simple ending, grandmaster Lajos Portisch made a technical mistake that turned the World's moral triumph into a mere relative success. First he missed the move that would have forced Korchnoi to resign at once: 57...Kh6. Then, a few moves later, wishing to gain some time for deliberation, though he was far from being in time pressure, Portisch repeated the position in the belief it was only a second repetition. But in fact, he had allowed a threefold repetition. Korchnoi, of course, claimed the draw.
Not everyone agreed on the analysis.
Reshevsky suggested 24. Rf1 Bx6 25. g3=.
Reshevsky gave 25...Re7 a question mark, adding that 25...Qe3, with the double threat of 26...Bg3 and 26...Nh5 with mate to follow in both cases. If 26. Nc4 Rxc4 27. Bxc4 Nh5 28. g4 (28. Bxf7+ Kh8) Qxf3 and wins. If 26. g4 Bc6 and the white Queen has no good square.
Reshevsky gave 57...Nf7 two question marks (Evans settled for one), stating "Portisch misses an easy win: 57...Kh6 58. Nh3 (if 58. Nd3 h3 59. Nxe5 h2 60. Re1 Rg2+ 61. Kh4 Rg1 wins) Rg2+ 59. Kxh4 Rh2 60. Kg4 Rxh3, etc."
Evans and Reshevsky agree on 58...g5? (58...Re2 59. Kxh4 Nd6 still wins)
Evans says 59...Ra8? was the last straw: 59...Re2! 60. Rf5 Re3 61. Kg4 Kg6 still conquers.
Reshevsky (after 63...Ra6): "Even though he is two pawns ahead, Black can make no headway: 63...Rf1 64. Kh5 Re1 65. Nxg5 Nxg5 66. Kxg5 Rxe4 67. Rb7+ Kg8 68. Kg6 Kf8 69. Kf5 Re3 70. Rh7 and draws."
OK, now to round two:
Portisch vs Korchnoi, 1970