What are some examples of wins by a much weaker player? I'd like to study examples where a much weaker player succeeded against an IM or GM.
In 2009, GM Nurlan Ibrayev of Kazakhstan (rated 2407 at the time) lost a classical time control game to 9-year-old Hetul Shah of India, who then had a FIDE rating of 1817. Shah's current rating, now three years after that game, is 1979, not so much higher than it was at the time of his upset victory. So this game wasn't a case of Ibrayev running up against some seriously underrated prodigy whose rating was about to shoot up several hundred points. To the contrary, this seems to be exactly the sort of game you are after: a much weaker player having a good day and taking a grandmaster scalp, with the black pieces no less!

[FEN ""]
[Event "7th Parsvnath International Open"]
[Site "Delhi"]
[Date "2009.01.11"]
[EventDate "2009.01.11"]
[Round "1.36"]
[Result "0-1"]
[White "Nurlan Ibrayev"]
[Black "Hetul Shah"]
[WhiteElo "2407"]
[BlackElo "1817"]
[PlyCount "88"]

1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 d6 8.c3 O-O
9.h3 Na5 10.Bc2 c5 11.d4 Qc7 12.Nbd2 cxd4 13.cxd4 Bd7 14.Nf1 Rfc8 15.Ne3 Nc6
16.a3 Bf8 17.b3 Nxd4 18.Nxd4 Qc3 19.Ne2 Qxa1 20.Qd2 d5 21.b4 d4 22.Nd1 Rxc2
23.Qxc2 Rc8 24.Qd2 Nxe4 25.Qd3 Nd6 26.Bb2 Qa2 27.f4 Qc4 28.Qb1 d3 29.Ne3 Qe4
30.Nc3 Qxf4 31.Ncd5 Qg3 32.Nf1 Qg6 33.Bxe5 Nc4 34.Nf4 Qb6+ 35.Kh1 Nxe5 36.Rxe5
Bd6 37.Re4 Bc6 38.Qc1 Qb7 39.Re1 Bxg2+ 40.Nxg2 Rxc1 41.Rxc1 Bf4 42.Rc3 d2
43.Rd3 h6 44.Kg1 Qb6+ 0-1