A close friend and I were on Fable II's colloseum mode. The two of us had played Fable II for a long time yet we never managed to do one thing on it. So the two of us came up with a crackpot crazy plan. Now for those who don't know about the Fable II colloseum your goal is to see how many rounds you can go in a set amount of time. Well using the time manipulation spell, we were able to actually BEAT the whole thing which we thought was similar to the Zombies mode in the Call of Duty games and was simply an infinite round thing. When we beat the Necromancer we finished it and sat back and were utterly disappointed that we finished it. Basically I have never, even to this day have felt such utter disappointment when doing something in a game I absolutely love. It's crazy when that kind of stuff happens where you think there is so much more before you are shown that no, you did it. It was crazy and I felt just... unaccomplished and for a Fable game, which are good at making you feel like you did something, it was crazy and completely different than what I was expecting.
Not sure if that is what you mean by crazy, but it was certainly a "what the f***?" moment type of crazy thing to happen.