When playing any opening, it is very important that you know how to play your opening especially razor sharp openings where the the line between winning and losing is on a knife's edge. Opening traps are abundant and one slip is usually fatal (even an incorrect move order can be detrimental).
Take for example, this miniature game (I'm White) in the French defence.
1. e4 e6
2. d4 d5
3. Nd2 Nf6
4. e5 Nfd7
5. c3 c5
6. Bd3 Nc6
7. Ndf3 Be7
8. h4 O-O??
All is peaceful, yes? No..........
In this case, my opponent didn't know the French Tarrasch defence well enough and what follows was .....
9. Bxh7! Kxh7+
Now all variations after
10. .... Bxg5 11. hxg5+ Kg8 or Kg6 12. Qh5 and it's game over
10. .... Kg6 11. Qd3+ f5 12. exf6+ (en passant) Kxf6 13. Qf3+ and it's not even funny
10. .... Kg8 11. Qh5 Bxg5 12. hxg5 and White mates the French like what Kramnik probably does everyday