I lost a horrible game in round 2 after missing a opening tactic, but for some reason my round 4 opponent avoided playing the same line. Instead he thought he would avoid my previous opening choices, but instead played into a Gruenfeld, which is an opening I have played for about 15 years.
But rather than show that game (which ended in a draw) I'll show an example where prep worked to deadly effect. Harry Press noticed his opponent played the same line of the Veresov, and after some investigation found a nice idea involving Ba3!!. His opponent started to go wrong straight away, and after another 5 moves resigned a he was about to lose a rook.
Salewski,Bernd - Press,Harry [D01]
London Chess Classic Under 2050, 15.12.2016