So there was a composer who wrote an opera, and this composer was so lackadaisical that he didn't finish the overture until literally hours before the first performance. This is what happens when you put off important jobs to the last moment, so here is the resulting overture. Listen and mock the poor composer's memory!
Oh, did I not name the composer? It's Mozart, and the opera in question is Don Giovanni, one of the operatic masterpieces of all time.
Mozart finished this overture the day before the premiere.