A long time ago when I was in high school, I worked at a Japanese restaurant as a busboy. Back then sushi was less common and kind of expansive. The servers would eat the left over sushi from the plate after they brought the plates to the back.

And yes they actually do things to the food if they recognized someone who didn't tip before. I remember one time they all got together and try to decide was someone in one of the table didn't tip last time.