Ah man mine was a multiplayer hotseat championship I had with a friend on Grand Prix 3 on the PC back in... 2001?
We were (still are) both massive F1 nerds and really went to town creating our own car texture to import into the game along with our own helmets. Each race was set to 50% and over a period of three months we'd go to each other's house carrying the save file with us. It got super competitive - each of us testing in our spare time to perfect set ups before each race and the like. As two kids who wanted to be F1 drivers when they grew up, we were kind of living our dream in a silly way.
Anyhow, the nature of hotseat mode on Grand Prix 3 meant that you drove for a portion of the Grand Prix and then handed control over to the other player, with the AI taking over your car and running at something approximating your pace. We got down to the penultimate round of the championship with him 6 points ahead of me and me really needing to beat him to take the title race to the last round. We'd had some epic races during the season and it was super-close - I can still remember some of those races even to this day. He'd jumped out to a big early championship lead but I had turned the tables with some really great drives in the second half of the season.
I'd had no time to perfect a setup in the week prior to the penultimate round but somehow pulled an amazing qualifying lap out of nowhere to take pole. It then rained on race day and I was notoriously crap in the wet, but off I sailed into the soggy distance driving the race of my life. By the end of my first stint I was about 4 seconds ahead. My mate takes over and my AI driver continues at a good clip, whilst he spins. Drama! If things stay the way they are I am about to turn a 6 point deficit into a four point lead heading into the last race. I drive the second of my two human-controlled race stints and hand back to my mate with a lead of about 15 seconds from the second placed car. I'd driven the race of my life and was really proud of myself.
My mate was left to drive around the sodden circuit to the chequered flag. He's made his way back up to third, so we will be heading into the final race equal in points which is pretty exciting stuff. He goes to approach the final chicane before the last corner on the final lap. Yellow flags! I squint at the monitor to try and ascertain what was happening. That isn't... it couldn't be... oh NO, it is...
... it was my car, beached in the gravel. On the penultimate corner of the last lap and with a 15-second lead, the AI version of myself had stupidly tried to lap a backmarker, tagged it and spun off into oblivion. It then sat there as every single car sailed past, as the AI on Grand Prix 3 tended to do in such situations. My epic win had been snatched away and not only that, by overtaking me and taking the flag in second, my mate wrapped up the championship then and there. We both kind of sat there staring at the screen in stunned silence. Months of practice, racing, banter, strategic planning all concluded by a foolish AI manoeuvre neither of us could do anything about.
At least it was memorable, I suppose!