Season 5 episode 1 “Homer’s Barbershop Quartet”

This is when the quartet is first getting started playing shows at Moe’s Bar. The joke is that right after Homer says that “the crowd went wild” and you see a bunch of flowers and women’s underwear being thrown on stage, it cuts to show that the whole “crowd” is just Barney. He waves his handkerchief at them and says “yoo-hoo” coquettishly lol

I’m surprised it’s not the “We want Chilly Willy” scene.