IIRC he forgot a note one performance and put the sheet music in front of him ever since. It is easy to imagine forgetting which high or low disconnected note

IIRC he forgot a note one performance and put the sheet music in front of him ever since. It is easy to imagine forgetting which high or low disconnected note you're supposed to start a passage from on a piece you haven't played recently -- or you have the sense of the feeling of a note but might be off by a fourth.