Chess-induced apoplexy - from crushing to crushed in one fell swoop
Here I was winning, nay crushing, an opponent rated several hundred rating points above me, having (bizarrely) won his queen early on in the game through a series of unimpeded knight moves, four in a row. It didn't last.
The white knight ensconced itself on d5 on move 12. It was irritating, but no more than an irritation for the next 17 moves. It stayed there until white's move 29, after I had blundered spectacularly on move 28 and turned a won game to a lost game in one fell swoop.
Turns out chess can induce apoplexy.
I should point out that this was a 3-day per move correspondence game. No blitz error, this.