While I can't swear the following is true, I have heard it from multiple sources through many years, with enough consistency and detail to believe it is true.
I never really knew any of them, but there were three great uncles, on my father's side. Kurt, Sven, and Wilhelm were all from the "old world", all classically trained, concert musicians. I believe it was Sven who was the viola player; He had a long and successful career in Southern California, playing with the L.A. philharmonic, and doing session work for movie and record studios. He worked on many of the major Frank Sinatra records, and frequently told of just how thoroughly professional "Old Blue Eyes" was. The man would often come into the studio three minutes before recording was to start, and crank out six or eight tracks in a row.....usually in a single take, bang on the money. No mistakes, no retakes, no ****ing around.
The musicians who backed him up were selected precisely because they could keep up; Place a written score in front of them, and they were expected to be able to read the music, and perform it without any warm up or rehearsal.
When Frank Zappa went shopping for musicians on one of his projects, it was that very quality that led him to my great uncle. Sven was handed his sheet music for the session, and was quickly stumped. He studied it for a while, then finally had to ask......"Excuse me, Mr. Zappa......there must be some mistake, I don't believe I can play this for you; I'm a viola player, and the music you have given me appears to be something written for a flute player, perhaps a piccolo?"
And Frank Zappa said... "Oh, when I wrote it, I
knew you couldn't possibly play it.....I just need you to
try and play it, do your very best; That will be the exact sound I'm after."
Edit: On review, the story likely leaves the impression I am unimpressed by anything less than total mastery, ability to sight read; To the contrary, I also have the utmost respect for the primitive and spontaneous; The undisciplined but brilliant.