I love the mechanics of the game. As such, I LOVE it when an NPC pulls off something completely unexpected for his archetype. Although I'll try not to let it influence my behavior in game, I'll obsessively try to puzzle out possible builds that would have allowed for the observed in game effect.

Much the same, but for when I DM, nothing thrills me more than hearing my players out of game puzzling over just exactly how the hell that happened in a given session. Always puts a smile on my face