The entirety of Granny Weatherwax's turn as "Lady Esmeralda", culminating in Nanny Ogg's... special... chocolate pudding for desert.
The villain is stabbed in a sword fight (actually, the sword is under his arm, but everyone's so wrapped up in the madness of opera that he dies anyway). He staggers to centre stage, delivers a long death soliloquy and dies. Then he gets back up to decry the opera some more, and dies. Then he gets back up again, and again, and again, reaches the five-exclamation-mark threshold of madness and declares:
"You know what really gets me down is the way everyone takes such a long!!!!! time!!!!! ... to!!!!! ... argh ... argh ... argh ..." *dies*
Nanny explains opera to Granny.
Nanny: Well, basically there are two sorts of operas. There's your heavy opera, where basically people sing foreign and it goes like 'Oh oh oh, I am dyin', oh, I am dyin', oh, oh, oh, that's what I'm doin'', and there's your light opera, where they sing in foreign and it basically goes 'Beer! Beer! Beer! Beer! I like to drink lots of beer!' although sometimes they drink champagne instead. That's basically all of opera, reely.
Granny: What? Is that it? Either dyin' or drinkin' beer?
Nanny: We-ell, there might be some other stuff, but generally it's stout or stabbin'.
The opera ghost wrote 'An opera about cats? Whoever heard of such a thing?' and 'An opera about a bloke called Les who's really miserable'.