While the person sitting across the board from you is nominally your opponent, most of the time the person you are really playing against is yourself. This is because when you are trying to calculate a move, you are also trying to find the best moves for your opponent. So when things go horribly wrong it is usually because what you think must happen, turns out not to happen.
Here is an example for the ANU Chess Club last night. Adrian de Noskowski won a pawn on move 21 forcing Mario Palma to try and defend the position a pawn down. By move 43 they reached the diagrammed position, when suddenly de Noskowski came up with a plan to mate the White king. Unfortunately the plan had a major flaw, which can happen with both players short of time and only receiving a 10s increment per move.
Mario Palma - Adrian DeNoskowski
ANU Autumn Swiss (2), 07.05.2008
1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.cxd5 Nxd5 5.e4 Nxc3 6.bxc3 Bg7 7.Nf3 c5 8.Bc4 0-0 9.0-0 Nc6 10.Bb2 Bg4 11.d5 Ne5 12.Be2 Nxf3+ 13.Bxf3 Bxf3 14.Qxf3 c4 15.Rfe1 Qa5 16.Re3 Rad8 17.Qe2 Qc7 18.h4 b5 19.a3 Qe5 20.Rf1 Bh6 21.Rh3 Rxd5 22.f4 Qe6 23.Bc1 Rdd8 24.f5 Qb6+ 25.Kh1 Bxc1 26.Rxc1 Qd6 27.h5 Qd2 28.Qf1 Qg5 29.h6 Rd2 30.Ra1 Rfd8 31.a4 Qg4 32.fxg6 fxg6 33.Qf3 Rd1+ 34.Rxd1 Rxd1+ 35.Kh2 Qxf3 36.Rxf3 bxa4 37.Rf2 Rc1 38.Ra2 Rxc3 39.Rxa4 Kf7 40.Rxa7 g5 41.e5 Re3 42.Rc7 c3 43.Rc5 Kg6 (D)
44.Rc6+ Kh5 45.Rc7 Kh4 46.Rxe7 g4 47.g3+ Rxg3 48.Rxh7 Rd3 49.Rc7 Rd2+ 50.Kg1 c2 51.h7 Kg3 52.Rc3+ This is the check that saves White. 52. ... Kf4 53.h8Q Rd1+ 54.Kf2 g3+ 55.Ke2 g2 56.Qf6+ Kg4 57.Qf3+ Kh4 58.Rc4+ Kg5 59.Rg4+ 1-0