Collect all of the Red Stars in Hard Mode to unlock some secrets. The sky changes pink, there are Kirbies you can control in the opening scene, and a free-play mode will be available for mini-games.
Unlimited lives
When you start level 4-3, you will see four Waddle-Dees walking around in a circle. Flipped, they will turn into blue stars. Collect all four of them to earn a life. Repeat this as many times as needed.
Invincibility
Set up a red bumper exactly vertical or horizontal with the bumper. Repeatedly roll into the bumper to become temporarily invincible. Use the blue glove power-up to get pushed in the air three times consecutively to become temporarily invincible. Enter a jumper, then jump out of it and into another jumper. Immediately jump out again into another jumper. Jump out once more time to become temporarily invincible.
