During the World Tournament Saga, when Gohan asks Goku to get a Senzu Bean, fly to Korin's Tower and get a bean from Yajirobe. Instead of giving it to Gohan, eat it. When you return, Goku will still give a Senzu bean to Gohan even though he does not have any. When you look in your invintory, you will see that you have 255 senzu beans.