Here's my recent loss to Matt Nesci. I told Cliff Kharoubbi that if I keep losing to his students I'm going to have to take lessons from him myself.
5...a6 The Sicilian Taimanov.
6. Nxc6 6. Nb3 Nf6 7. Bd3 d5 8. exd5 Nxd5 9. Nxd5 Qxd5 10. O-O = Trowbridge-Phillips, CCCR Championship Trnmt 2013. 6. Be2 and 6. Be3 are the more usual moves.
7...Bb4 7...Qc7 is more common. For example 8. Qe2 Rb8 9. g3 Ne7 10. Ne4, Svidler-Cossin, FRA ChT 2009.
8. Bd2 The computer likes 8. Qg4! and 8...Bxc3+ 9. bxc3 Ne7 (or 9...g6 10. Bd3+-) 10. Qf6+-.
9. a3 I seem to throw away pawns very early in the game. It might be because chess comes at the end of the day and I'm tired. On the other hand maybe I'm just undisciplined. During the post-mortem Kharroubi was completely unconcerned. "So? Grandmasters gambit pawns away all the time!" That, my friends, is chess wisdom.
12. Be3? Another oversight. 12. Bxg5 wins the pawn back as 12...Qxg5 and either 13. Qxd6 or 13. Ne4.
17. Bd3 to prevent the forced exchange of another piece by ...Ne4.
23. Qe3? A final and decisive mistake. I missed 23. Rxe6+! fxe6 24. Qg7+ Kd8 (24...Ke8 25. Bg6+ is lost for Black) 25. Qxh8+ with a strong advantage for white. 23. Rxe6 is a difficult sacrifice to find, although probably easy for 1900+ players.
23...Bc6 The computer prefers 23...Qc6 with a strong advantage but black has to find some "only" moves or he may allow white to equalize, e.g. 24. Be4 and now A) 24...Qxa4 25. Bxb7 Rb8 and 26. Bd5 or 26. Bg2 equalize; B) 24...Qxe4 25. Qxe4 Bxe4 26. Rxe4 d5 27. Re2 and black keeps his pawn advantage but white has chances; and C) 24...d5 is probably best but it's tough to find. 25. Bg2 Qxa4 and now white has to find 26. fxg5. If then 26...hxg5 white has 27. Rxf7+! Kxf7? 28. Qxe6+. The line is beautiful but it's not likely either of us would have found those moves.
25...Qb6! The forced exchange of queens practically wins for black.
29. f6+ I spent a long time on 29. fxe6 deciding which one might give white more opportunity for "tricks". They both looked losing however, and indeed the rest of the game is lost for white.