Select to play as Sonic and Tails and enable the 'Level select', 'Super Sonic', and 'Debug mode' codes and enter any stage. Get 50 rings and turn into Super Sonic. Change Sonic into the television monitor. Change Sonic back and have him jump to turn into Super Sonic. Jump on the television monitor to have Tails become invincible. Then, turn into an enemy and put it right on tails while he is invincible. Continuously place the enemy there and since Tails is locked in the sonic spin move and he will destroy them. As Tails destroys the enemies in this position keep pressing C (don't let go of Down on the D-pad) until the score for destroying an enemy increases from 100 to 8,000 to a 1up. When you have enough 1ups, press B again to revert to Sonic.
This feature is enabled by activating 'Level Select' and holding C while selecting a stage. Entering this code will darken the level but the effect reverts to normal when you die or when you beat the level.
To do this go to the sound test (not the one on the level select) and put in 19, 65, 09, 17, 01, 01, 02, 04 making sure you press A after each one (there isn't a confirmation sound). Press 'Start' on the first option ('Character' select) to begin the game and you will have 14 continues if the code was entered correctly.
Change Tails' Name to MilesAdded 26 Apr 2011, ID #3838
At the 'Title' screen press Up, Up, Up, Down, Down, Down, Up. If you have entered the code correctly you will hear a confirmation sound and Tails' name will be changed to 'Miles'. Note: The opposite will occur in the Japanese version of the game.