Disclaimer: I make no claim. I make no money. Hopefully, this ensures I make no court appearances.
Timeframe: Almost anytime in the first three seasons.
'Well, that was kind of funny, if you think about it.'
'Xander. If you do even think about it again, I will hurt you.'
'Oh, c'mon, Buff. You aren't seriously going to try and tell me it's never happened before. And it helped.'
'Helped? What do you mean it helped? How could it have helped?'
'Well, he did kind of have you on the ropes there. And I still hadn't regained the ability to stand upright, so my patented Ineffectual Suicide Attack wouldn't have been available.'
'I Slayed him, didn't I?'
'Well, yeah. But only because your top came off.'
'If you mention this ever again, to anyone, I will kill you.'