Chess Tactic - Can you spot the mistake?

I was browsing Grandmaster Nigel Short's game database on http://www.jrobichess.com and GM Peter Nielsen played Ng6 against Short in this 2008 Chess Olympiad game - can you spot the problem with this move? What do you think the plan was before he played it?

[Event "2008 Olympiad"]
[Site "Dresden GER"]
[Date "2008.11.14"]
[Round "2"]
[White "Nigel Short"]
[Black "Peter Heine Nielsen"]
[Result "1-0"]
[BlackElo "2662"]
[ECO "C42"]
[EventDate "2008.11.13"]
[PlyCount "91"]
[WhiteElo "2642"]

1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. d3 Nf6 6. d4 d5 7. Bd3 Be7 8. h3
Nc6 9. a3 O-O 10. O-O Re8 11. Nc3 h6 12. Re1 Bf8 13. Bf4 Rxe1+ 14. Qxe1 Bd6 15.
Qd2 Be6 16. Re1 Bxf4 17. Qxf4 Nh5 18. Qe3 Nf6 19. Ne2 Qd6 20. Ng3 Ne7 21. Ne5
Qb6 22. f4 Nc6 23. c3 Re8 24. b4 Ne7 25. f5 Bc8 26. Ng4 Nxg4 27. hxg4 Qd6 28.
g5 hxg5 29. Qxg5 f6 30. Qh4 Bd7 31. c4 Nc8 32. Rxe8+ Bxe8 33. c5 Qe7 34. Kf2 a6
35. Ne2 Bb5 36. Bxb5 axb5 37. Qf4 Qd7 38. g4 Ne7 39. Qf3 Kf7 40. Nf4 c6 41. Qh1
Kg8 42. Qh5 Qc8 43. Kg2 Qb8 44. Ne6 Qc8 45. Qh2 Ng6 46. fxg6 1-0

jrobichess makes chess videos and has a chess blog along with a personal chess site at http://www.jrobichess.com.

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jrobichess makes chess videos and has a chess blog along with a personal chess site at http://www.jrobichess.com.

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jrobichess makes chess videos and has a chess blog along with a personal chess site at http://www.jrobichess.com.