This isn't quite a greatest moment, but I was playing with my dad in a Colony game and I was going for something that would probably take a bit too long in a normal game, but I thought that in a Colony game it would be perfect. Part of the way through, I realized we hadn't played with Colonies for a while so I reminded of the game-ending rules. He realized I was planning something and rushed the Provinces (being reminded they can end the game still), RIGHT before my thing was going to kick in. He was gloating on his 67-17 victory a bunch.
So, we played another game. And, to get him back, I made King's Court, Goons, and Masquerade in the next game... Anyway, when I pulled it off, he resigned (and I don't blame him). He realized that I did it on purpose, and we both were laughing and said it didn't count as an actual game.