I can't understand why it occurred only on piano sound... When it happened, I set the Stage Setting 2-0 (don't remember the name, but it's a factory preset) and, instead piano + strings, I heard only strings. Immediately I set 0-0 and didn't hear the piano. Master Volume isn't zero and no sliders or pots were moved. It's very weird. I use the key twice a month in a church near home, so vibrations are out of question.