Temple Key

Plantera drops it and the only thing it opens is the temple entrance. It is spent the moment you use it, and once that door is unlocked it behaves like any other door you can open by hand, so you never need a second one. Extra copies from repeat kills are dead weight: they cannot be sold and nothing is crafted from them.

The lock only works on a complete frame. Strip the block above or below a Lihzahrd Door and the key stops working on it, and since the door itself cannot be broken by a pickaxe or explosives either, you are left with a fixture nothing in the game can remove. It is a curiosity rather than a problem, but it is worth knowing before you start reshaping the temple entrance.