If you could de-invent anything what would it be, and why?
Mine are a toss-up between the Atom bomb and personal bloody stereo's.
The atom bomb, because there would have been no Hiroshima and Nagasaki, and probably no development of today's nuclear weapons, and all the potential for mass destruction that go with them.
The personal stereo because they are just so flipping irritating when you get someone else's crap choice of music second hand.