Pianist Roger Woodward in his memoir, Beyond Black and White, (2014, Kindle Edition, location 5265):

Violin virtuoso George Enescu and acclaimed pianist Arthur Rubinstein were once lunching together at Enescu’s Paris apartment when there was a knock on his door. It was one of Enescu’s violin students, who was beside himself. Enescu tried to calm him down as the student explained that his piano accompanist had telephoned half an hour earlier to say he had fallen ill and was not able to replace him for his debut that evening. Since Enescu was as fine a pianist as violinist, he reassured his student he would accompany him. Rubinstein volunteered to turn pages. Next day, one of the Paris critics described the concert in the following way: “Except for the fact that the pianist should have played the violin, the page-turner the piano, and violinist the pages, the concert was memorable.” “

Woodward tells this story when recounting a rehearsal of Iannis Xenakis’ Eonta in UCLA in March 1972 under Zubin Mehta at which Olivier Messiaen and Yvonne Loriod turned up unexpectedly, having braved rioters and police on the UCLA campus to get there. Eventually Messiaen turned Woodward’s pages for him. However, Messiaen, who had been Xenakis’ composition teacher and knew his music (and who was always supportive of his student), first helped the brass players (from the LASO) deal with the many challenges and mis-prints in their parts for Eonta.