Red's armor

Red's Armor is an easter egg vanity set designed and for use by Redigit, the head programmer of Terraria. Its tooltip reads "You shouldn't have this", and without the use of inventory editors it is indeed impossible to obtain.

Trivia

 * The phrase "You shouldn't have this" may be a reference to Ben Drowned, a Zelda creepypasta of a haunted cartridge of the game Majora's Mask. This has not been confirmed.