This is a re-upload; since YouTube removed annotations, I had no choice but to remake it. You could see the original text for a split second at 0:12.
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https://www.youtube.com/watch?v=gsMOaXu5AW0
How this glitch works: In the Tropical Island Secret Star course, Get your health low to only one pie. Then, get hit by the koopa troopa that is near the star sprite. Make sure you get hit high enough so that the player will get the star sprite. If done right, the player will die instead of receiving the star sprite. You will then lose all of your lives and end up in the Main Hall of the castle. If you didn't beat the stage, you'll have to restart all over again.
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https://www.youtube.com/watch?v=TLnYeIq1HXQ