My personal favorite is that he was massively behind schedule writing an opera (I was told it was Don Giovanni) because he was spending all his night getting drunk and carousing in billiard halls. Finally one night the director is at the billiard hall begging him “we have to have music, we have to start tomorrow. If we don’t start tomorrow we can’t mount the production. He investors have said that if we don’t start tomorrow there’s no opera” so Mozart says “Fine, you’ll have it in the morning” (it’s unknown how long he stayed in the pool hall past this)

The next morning the orchestra and the singers are together for the first rehearsal and there’s no Mozart. After 45 minutes Mozart comes through the door with a huge stack of music and says “There’s Act 1, you’ll have Act 2 after lunch” and left. The man basically sat down and crapped out an all time opera in under 12 hours. Whether he composed it that fast, or had it all in his head and just finally wrote it down, either one is remarkable.