That's the way top level (or even club level) chess is, where a single blunder, no matter how good your previous moves have been, can lose you a game in an instant; all that mental effort you've expended over the course of the game just wasted.
Blunders in chess come from a variety of sources: mental fatigue, the pressures of a ticking chess clock (as happened to Nigel Short), impatience, or simple oversight.
Rarer is a combination played in the wrong order, but it happens. You calculate a winning combination, check a few other lines, then go to play your 'win'. Only you don't! In your haste you play the moves in the wrong order or simply forget to play the first move. This position from the Paris Championships back in 97 is a case in point:
After a double edged middle-game, I'd gained the advantage with White against Maria Nepeina-Leconte, a Ukranian International Master. I thought for a while, saw a winning combination, checked my analysis, and went to play the line. I played my first move correctly, but then managed to mix up the variations in my head and hastily played the wrong second move:
1. Ng6! (Ne6!! is even better) fg 2. Qxh6??
I had calculated and intended to play Rf8+ which wins simply, but for some reason my hand moved the Queen.
2. ... Bf5 3. Rxf5?
White could still force a draw with 3. Rb3!, but now Black is winning.
3. ... gf 4. Qe6+ Kg7 5. Qd7+ Kh6 6. Qxf5 Re1??
Which allowed me to escape with a draw by perpetual!
