I've played in a number of masterclasses conducted by great pianists and pedagogues (to name a few)- Menahem Pressler, Rebecca Penneys, Ann Schein, Gyorgy Sebok (shortly before he passed away) - he was improvising on the second piano while I was playing my Schubert sonata in D major!.
I have heard these pianists' live performances: Helene Grimaud, Constance Keene (turned page for her and received a free CD!), Andras Schiff (later asked for his autograph on his Bartok concerti recording), Chick Corea, Emmanuel Ax, Hamelin (took photos with him afterwards!).
I remember meeting the cellist, Yo-yo Ma, after his concert and asking him to sign an autograph. And he signed it upside down while standing face to face with me, but his writing appeared right side up to me! Amazing!!