According to Frederic Friedel, Kasparov's second, Garry was very
tight-lipped and tense all day prior to the game; certainly not
the temperment that facilitates competitive play.
It takes more than one bad move to lose a chess game;
it takes several, or one whopping blunder.
The World Champion quickly replied 8. .. Qe7,
which is another known mistake.
Black's hopes must ride on 8...fxe6 9.Bg6+ Ke7 and setting the king up
on d8, the queen on c7, and developing the dark-squared bishop to d6.
Black's game is still extremely difficult to develop while
his king is buried in the center,
but at least White's immediate threats are muted.
According to Seirawan,
"Deep Blue's operator, Joe Hoane, was so nervous at this point
[...] that he shakily started playing 10. Bc1-g6+, an illegal move.
Since this is not Deep Blue's fault the rules concerning the
operator's moves are not touch move."
The audience must have howled at the suddent appearance of
two queenside bishops for the White army.
After 11. Bf4 and 12. a4 Deeper Blue established control over the
Black king's escape route, and looked forward to a quick decision
on the royal man himself.
17. Bf5!!, forcing the win of material, was followed by the final blow
19. c4. Kasparov resigned in the face of mate by Qa6 if the pawn is captured,
or a hopeless material deficit after 19...Nb4 20.Qxf5 Rf8 21.Qe6 bxc4 22.Ne5.