Hi
it's a lot more common that you would think.
Blunders & Brilliancies a very good book by Ian Mullen & Mo Moss
has a whole chapter dedicated to players, some quite strong,
resigning in won positions.
One (not in the book) is legendary in Edinburgh.
James 'Two Queens' Turner resigned as Black in this position
in the 2008 Edinburgh Congress.
Here is the full Game and I've contunied the game from the
position in the diagram to show the win. It's quite an entertaining game.
[Event "Game from non-PGN file"]
[Site "END-CFB10B3985E"]
[Date "2009.09.12"]
[Round "-"]
[White "?"]
[Black "?"]
[Result "*"]
1. d4 d5 2. e3 Nf6 3. Bd3 c5 4. c3 c4 5. Bc2 e6 6. Nf3 Nc6 7. Nbd2 b5 8.O-O Be7 9. Re1 Qc7 10. e4 dxe4 11. Nxe4 O-O 12. Bg5 Nd5 13. Bxe7 Ncxe7 14.Ng3 Nf6 15. Re5 Ned5 16. Nh5 Nxh5 17. Bxh7+ Kxh7 18. Ng5+ Kg8 19. Qxh5 Nf6 20. Qh4 Rd8 21. Rae1 Rd5 22. R1e3 Bb7 23. Rxd5 Bxd5 24. Re5 a5 25. f3 b4 26. Qe1 bxc3 27. bxc3 Qb6 28. Qd2 Rb8 29. Re2 a4 30. Ne4 Bxe4 31. fxe4 a3 32. Qe3 Qb1+ 33. Re1 Qxa2 34. e5 Nd5 35. Qf2 Rb2 36. Qf1 Nxc3 37. h4 Qb3 38. Kh2 a2 39. Qf3 Rb1 40. Qa8+ Kh7 41. Re3 a1=Q 42. Qf3 Qaa3 43. Qxf7 Qaa4
44. Rg3 Rh1+ 45. Kxh1 Qa1+ 46. Kh2 Qg1+ 47. Kxg1 Ne2+ 48. Kf2 Nxg3