The 13 Star-Coin Paradox

"'By collecting 13 Star Coins, a Time Paradox appeared; you now have -3 Shine Sprites... Just kidding... Instead, you've unlocked the Haunted Pack (Enemies, Tileset, and Song) for the Level Designer.'"

This occurs when you get 13 Star Coins. Although there is no time paradox, it is still pretty fun to do this!