I've always wanted one of the flexible mannequins sold by TK Products, the same vendor that makes the iComm stormtrooper voice box. But with shipping and import taxes to Norway, it is just too expensive for me. Heard a lot of good things about it, though.
Not a prank, but a tool. Duct tape mannequins are very useful during a costume build, especially if you need to do lots of testing and measuring of costume pieces. Since the mannequin retains the shape of your body, you use it to fit the pieces on the mannequin instead of yourself. Easier, more comfortable. But it isn't very durable for display purposes.