Ignaz Xaver Seyfried was asked to turn pages for Beethoven in a performance of Beethoven’s Third Piano Concerto (5 April 1803). He recalled:
In the playing of the concerto movements he asked me to turn the pages for him; but – heaven help me! – that was easier said than done. I saw almost nothing but empty leaves; at the most on one page or the either a few Egyptian hieroglyphs wholly unintelligible to me scribbled down to serve as clues for him; for he played nearly all of the solo part from memory, since, as was so often the case, he had not had time to put it all down on paper. He gave me a secret glance whenever he was at the end of one of the invisible passages and my scarcely concealable anxiety not to miss the decisive moment amused him greatly and he laughed heartily at the jovial supper which we ate afterwards.
Thayer, Alexander Wheelock, Elliot Forbes, ed. (1964) The Life of Beethoven, Princeton. Cited in: Marek, George (1969) Beethoven: Biography of a Genius. London: William Kimber, p.336-337.