that wouldn't actually be a contradiction. there's a stark difference between convincing people that something is true, and something actually being true. the equivalent of what you are saying is that george bush telling the world that saddam hussein posed an imminent threat to the west, with his weapons of mass destruction, that the fact that these weapons didn't seem to exist is somehow a paradox. in reality, he simply lied. there is a difference between a contradiction and a lie. the god scenario applied would mean not only that he 'convinced the world he did not exist while existing,' but that he existed while not existing simultaneously. as far fetched as this scenario may seem, it is one of the necessary functions of an omnipotent being, which has shown to be largely incoherent.