[Spoilers, I guess, if you haven't played Assassin's Creed II.]
I've been playing through Assassin's Creed II and really enjoying it, until I got to a point where I'm told to kill Rodrigo Borgia. Being a total badass with a wrist pistol, I go about doing just that, but every time I get close some other idiots get in the way, including, at one point, my "friends".
Then when I'm about to kill him he kicks me...and runs off. AND THEN I'm told that I shouldn't bother chasing him.
From that point on the rest of the game soured, especially when essentially the same thing happened again and some bastard stabbed me as I was killing him - me, the craziest assassin in Italy.
I was reminded of the ending of God of War II. You spend the entire game establishing that you are the king of all hurt, and when you finally have Zeus in your arms and are literally going to break his back, he...I can't even remember - flies away? bedazzles you? - and suddenly he's on Mt. Olympus, raising the question as to why he didn't do that in the first place.
And despite having the greatest time playing through that game, all I really remember is the non-ending. I get that III was already in the works, but when you spend the game establishing that you can and should kill anything, having a guy just run away is basically the gaming equivalent of "it was all a dream".
Anyone play anything else that had that kind of moment? Not necessarily the same mechanic, but a moment so jarring that it ruined an otherwise enjoyable experience? (And I'm exaggerating a bit with ACII, but not GOWII.)