 There are several Chaos Emeralds hiddein in bonus levels throughout the  game. If you collect them all, after the credits roll at the end of the  game, Dr. Robotnik will be seen jumping up and down over the words THE END.  However if you do not collect all the emeralds, following the credits will  be Dr. Robotnik juggling the ones you did not collect and standing over the  words TRY AGAIN.