Which itself is a folklore fail, actually. It's mostly what Mjolnir was intended to be, a two-handed warhammer, but Loki was determined not to let the dwarf brothers Eitri and Brokkr finish the best gift for the gods (after a boar with shining bristles for Freyr (Gullinbursti), and Odin's replicating golden ring Draupnir), as it would have cost him his head. So he transformed into a fly, and bit Brokkr on the eye hard enough to draw blood, which made him stop blowing in the forge (against his brothers order to never stop blowing until he returned), so Mjolnir wasn't fully formed in the dwarves' forge, making it one-handed. This makes Mjolnir itself a "forging fail". This is why handling Mjolnir necessitates using another one of Thor's crucial possessions, the Járngreipr/Járnglófi (Iron Gripper/Gauntlet).
Just to finish the story, Loki lost the bet, because Thor valued the hammer even with its shortened handle, and with the bet, Loki lost his head. He got to keep it though, because cutting it off wasn't included in the bet. So Brokkr sewed Loki's mouth up to teach him lesson about messing with the dwarves...
