I'd have to say that my biggest shock victory was in Baldur's Gate 2, fighting the dragon that's just before the ending (the one you HAVE to kill in order to get a quest item). In that game, the dragons were an all-or-nothing proposition, and the only really solid strategy I'd found for defeating them was to use Feeblemind and hope they failed their save.
Well, it didn't work, and it was mauling my party while I desperately tried to come up with plan B, and then, boom, the dragon suddenly vanished. Apparently one of my mages Disintegrated it. While I wasn't looking.
Oooookay then.