My opponent played an early Bg5 against my King's Indian Defense, and I forgot not only the line against it (Spassky-Fisher 1992) but the general approach to the Averbakh.
I ended up with an unfavorable Benoni setup, and had to defend against a strong kingside attack. Short on time, my opponent missed the killer move, and allowed me to give up a piece for 3 center pawns and transition into an endgame, where my central pawn mass proved to be too much trouble for him in the time he had left.
Recently I picked up an ideas I am going to try to apply now. The old idea was simply to study an opening after as part of analyzing the game just played. The new idea is to look at an opening 3 times in a day, with an hour or two break between first and second look, then more than four hours between second and third.