Super Tails
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Enable Level Select, then press
Down to make sure you have the sound test (don't select it yet). Start a one-player game and choose Tails in the Data Select screen. When the timer appears, press RESET.
Enter the Sound Test menu and choose any zone. Hold A and press Start. You should be Tails and be in debug mode. From here press B, then press A until you see a box. Press C to select it, then press B to return to Tails.
To enable Super Tails, jump on the box. Tails will look weird and can not be hurt or lose any rings. He can still get squashed and drowned, though, so be careful in the Ice Cap zone. You will have do this at the beginning of each zone. Robotnik can hurt him at the very end of Launch Base zone-act 2.