I have an invention which would have ensured many, many more fight victories for the Jedi Knights and for the wizards in Harry Potter's world:
A wrist-strap for their light sabres and their wands.
I can believe that a guy can fly on a broom, teleport, and lift objects with his mind. But it really pushes my suspense of disbelief to the breaking point that nobody ever thought to put wrist-straps on those things. They drop them in every dang battle they are in!