I found it. This was the one.
My wife punched me in the face once.
Well - more accurately, she gave me an uppercut to the chin. She'd never hit me before. She weighed around 39kg at the time and I'm not sure if she even put all her strength into it because it obvisouly didn't even come close to hurting.
She hit me, I looked at her, she looked at me, I couldn't keep a straight face and within seconds we were both howling with laughter. Mostly because we were both being dicks in the first place.
Anyway - it never happened again and we still laugh about how "the uppercut". I guess that's what comes of having a woman that grew up with 2 brothers....