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.