The story, according to Mike Fox and Richard James, in their The Even More Complete Chess Addict, is that Fischer has made his move, setting a trap in what is an objectively bad position, and has gone for a little walk while awaiting his opponent's move. Viktor Korchnoi and Tigran Petrosian are watching the game see the trap and discuss the solution. Petrosian's wife is there too, and as her hubby is trailing Fischer in the standings, she actually walks over to Kovacevic and whispers the solution to him! Kovacevic plays the right move and crushes Fischer, though the latter went on to win the tournament by a two point margin even so.
As far as I know, the source of this anecdote is Korchnoi, and in light of his long-time enmity towards Petrosian, it's possible that the story is bunk. Further, while Kovacevic's 18th move was a nice one, it wasn't beyond the capacities of a strong IM, later a GM, to find such a move. In any case, here's the game:
Fischer,Robert James - Kovacevic,Vlatko [C15]
Rovinj/Zagreb Zagreb (8), 21.04.1970
1.e4 e6 2.d4 d5 3.Nc3 Bb4 4.a3 Bxc3+ 5.bxc3 dxe4 6.Qg4 Nf6 7.Qxg7 Rg8 8.Qh6 Nbd7 9.Ne2 b6 10.Bg5 Qe7 11.Qh4 Bb7 12.Ng3 h6 13.Bd2 0-0-0 14.Be2 Nf8 15.0-0 Ng6 16.Qxh6 Rh8 17.Qg5 Rdg8 18.f3
Black's position is beautifully coordinated for a kingside attack, and if White doesn't do anything special Black will roll him off the board. White's last move is a nice try: he's hoping to shut down the Bb7's diagonal (pre-f3, Black threatened, among other things, some combination of ...Nh4, ...e3 and ...N/Bxg2) and to open the f-file for his own use. 18...e3! This keeps the f-file closed, and now, despite the two pawn deficit, Black's attack is unstoppable. [18...Nh4 looks crushing: the queen moves away, and then 19...exf3 destroys the White kingside. However: 19.fxe4! Rxg5 20.Bxg5 puts a stop to Black's attack, and after 20...Nf5 (forced) 21.Nh5 Rg8 22.Bxf6 Qf8 23.Rf4 Ne3 24.Ng3 the position is unclear.; 18...exf3? 19.Bxf3 is clearly better for White, as Black has serious problems along the f-file.] 19.Bxe3 Nf8! 20.Qb5 Nd5 Not just hitting e3 and c3, but cutting the white queen off from the kingside. 21.Kf2 a6 22.Qd3 Rxh2 23.Rh1 Qh4 24.Rxh2 Qxh2 25.Nf1 Rxg2+ 26.Ke1 Qh4+ 27.Kd2 Ng6 28.Re1 Ngf4 29.Bxf4 Nxf4 30.Qe3 Rf2
Black is threatening 31...Nxe2 32.Rxe2 Rxf1, 31...Ng2 and 31...Bxf3 - too much! 0-1