Turn Someone On from Monsterhearts 2 will always have a soft spot in my heart. Less because of its mechanical implications (which are still great), but because the you can trigger it without meaning to, and it changes the context of the scene in a heartbeat. I remember a game I ran where the characters were just friends, and one was tending to the other's wounds. Then from the healing move, you could heal +1 harm with romantic subtext, so I told them to roll to turn someone on, and boom they became the star couple of the game from then on (there was more drama than that but you get it). No one planned for it and it became the thing all my players still talk about even after the game ended.