Anyway- Hunted, the Demons forge. Coop. At several points in the game, you have this clearly evil cauldron of superpowers lying around. So at one point, I decided to use it, you know, just to see what its like. Turns out (according to youtube), if you do it, even just once, the game ending turns into "and then they were mindless slaves forever". Yeah, okay, you warned me (sort of), but still! Geeze! Never did finish that campaign because of that...
That's sadistic! Which reminds me, I'd love to add a sort of what happened afterwards sequence to Driftmoon...
My worst experience was a game called The Flintstones on the original NES. I saved money for weeks to get it, and then it was too difficult for me. Ah, the era of instant death by jumping into bottomless pits. I never got past the first two or three levels without cheats. A lot of NES games were like that, you could only finish them by practicing for months.
I think I've been burned by some sequels as well. Star Control 3 comes to mind, which was sad considering Star Control 2 is the best game of all time. And of course any Ultima after 7, though I forced myself to play them because I was such a big fan of Ultima 7.
Oh yeah, now I remember the worst one. XCom 2. My strategy was to take it slow, build up my forces and only take on missions I know I can handle. I played for weeks like that, until I was sure I could take on the most badass enemies. Turns out the number of alien bases had been increasing exponentially all the while I played my safe routine, so when I decided it was my turn to attack the enemies, there were about 500 alien bases now left to eradicate. I tried and I tried for many days, but the aliens now attacked my base every 2 minutes and still increased exponentially.