Zapinator

The Zapinator was a weapon that was accidentally left in the game when it was released, but was taken out in a subsequent update. It resembled an NES-era light gun and shot very rapidly, dealing 50 damage per hit - around 500 damage a second. This was a weapon that Terraria's creators used during testing without worrying about enemies. It can be obtained by defeating Plantera in the 3DS version, although its exact drop rate and best modifier is unknown.

Trivia

 * In terms of damage per hit, most Hardmode weapons do more damage.
 * The Zapinator's name and sprite resemble the Nintendo Zapper; A light gun peripheral accessory released with the NES.
 * The Zapinator's projectile ID was 20 (the projectile fired by the Space Gun.)
 * Unlike the real life counterpart, the Zapinator is not grey in the Japanese 3DS version (yet to be confirmed).
 * The reason why the Zapinator is in the 3DS version at all remains unclear. However, this is the first time Terraria has been on a Nintendo system, so this may be an intentional easter egg.