It aint over till the fat lady sings

A phrase used all the time now is 'it aint over till the fat lady sings.'
This makes no sense in operatic terms since the fat lady usually sang in the first act. I heard the phrase originally as: it 'aint over till the fat lady dies'. This is more usual in opera and makes sense. Does anyone else remember the original?

I can picture Moe waking Curly up with a sharp elbow to the ribs, saying "All right, the fat lady's dead; let's scram" but I'm having a hard time picturing a scenario where someone would teach a budding opera buff that the way you know when it's over is when the fat lady dies. Then again I don't know any opera buffs and maybe they're more earthy than I thought.

[It doesn't have to make sense to a wise opera-goer, but it makes perfect sense to the boyfriend or husband dragged along and just willing to put up with a couple of hours of such entetainment. Picture it through his eyes: curtain up, the whole company marches onto the stage, outlandishly costumed; he marks one, a fat lady in a white bathrobe with a gold rope around the equator; he doesn't know the word "supernumerary"; she's wearing a horned helmet and carrying a spear: she looks important; moreover she fits his image of an opera star. Everyone on stage is singing; he waits for her to step forward and go into her solo - but instead, the chorus ending, another cast of characters enter and start batting tunes back and forth. The chorus springs back to life, but only for a moment; then the fat lady scoots off and her pals with her. These other characters make eyes at each other; they have a lot to sing about, supposedly; sometimes they wave swords or pose on the picturesque rocks. The poor gentleman in the audience settles in for the duration; a blurry lifetime later, girlfriend or wife elbows him into alertness: he focuses on the stage. Is it over yet? No. "It ain't over till the fat lady sings." - Bac.]