Mjölnir

The hammer, the Prose Edda tells us, was born from a bet. Loki, caught cutting the golden hair from Thor's wife, wagered his own head that no dwarf could match the treasures of the sons of Ívaldi. Brokkr and Eitri took the wager — and Loki, who had staked his head on their failing, turned himself into a fly and bit the one working the bellows. He flinched. The haft came out short. Nobody corrected it. In the myth Mjölnir never misses and returns to the hand that threw it, and it is still too small to hold properly, which is why the sources give Thor iron gloves.