Mmmm. One of the funniest episodes (and the saddest, of course), and one of my very favourite episodes in the entire series, is Tabula Rasa.
"Hey! ...I mean, heyyyy."
"Randy Giles!!?"
"Oh, Rupie."

And that look on Tara's face.

But it's the embodiment of everything you're saying. It's only funny when YOU know who they really are, even though they don't.
It's only sad when you know the true extent of Willow's betrayal.
"Goodbye to you" still makes me cry!