I was quite pleased when Andrew Brown won his last round game at the 2011 Oceania Zonal to clinch the IM title (and second place). Because a number of years ago I lost a game to Andrew that made me consider whether I was even competent enough to keep on playing. It didn't have anything to do with Andrew mind you, except that he was on the other side of the board when I simply blew a winning position by blundering a piece with check.
The loss of that game created a real crisis of confidence in me that affected my play for the next few months. I began to worry about every move, making sure I wasn't making the same mistake, but such "second guessing" can affect how well I play. It took a few months before my results began to recover. Eventually they did (I even won against Andrew in the 2005 ANU Open), and while it wasn't a happy experience, I can now look back and say "Don't be so hard on yourself, he was a future IM after all"
Brown,Andrew - Press,Shaun [D03]
Belconnen Open Belconnen Chess Club, 11.02.2005
1.d4 Nf6 2.Nf3 e6 3.Bg5 h6 4.Bh4 Be7 5.Nbd2 d5 6.e3 b6 7.Bd3 Bb7 8.c3 Nbd7 9.0-0 0-0 10.Qe2 c5 11.a4 Ne4 12.Bxe7 Qxe7 13.a5 f5 14.a6 Bc6 15.Bb5 Bxb5 16.Qxb5 g5 17.Nxe4 fxe4 18.Ne1 Rf6 19.Qe2 Raf8 20.Nc2 Qf7 21.b4 e5 22.b5 Rc8 23.Rfd1 exd4 24.cxd4 c4 25.Nb4 Nb8 26.Qg4 Rf8 27.h4 Rg6 28.Ra2 gxh4 29.Qf4 Qd7 30.Qxh4 Qxb5 31.Qh5 Rd6 32.Qg4+ Kh8 33.Rb1 Qd7 34.Qh5 Rf5 35.Qh2 (D)
Rg6 36.Qxb8+ 1-0