I can't know the specifics of your anecdote, but Toscanini had a formidable
memory.  He was myopic, which was why he didn't use a score.  But there
is a story that I think is true about him reminiscing with some musicians
about his youthful studies, wherein he remembered a string quartet he had
played.  At that time he wrote out the quartet from memory.  I should
mention that it was by a composer unknown to his musicians.  One of them
looked up the music in a library, and found that he had made one minor
mistake from memory.

This story is often told in Toscanini biographies.

Bill S