Harpoon Launcher

The Harpoon Launcher is a mechanical weapon which was shown in the Adventure Time episode. In the episode, Finn tries to operate it but gets too confused by the codes and holograms displaying out of the machine. He gets distracted when Jake asks him to go see Jermaine together which results in the harpoon launching out of the machine creating a hole through the tree house.

Trivia
The harpoon is later revealed to have accidentally pinned a huge slug after it reached impact onto to ground.