Bad arguments against the existence of God
Bad arguments against the existence of God are those that most theists will easily rationalize their way out of. These may be logical fallacies, or paradoxes that theists can explain away.
Omniscience Versus Free Will
Some atheists will argue that since God supposedly knows everything, then everything is predestined. Since many theists do not accept the predestination doctrine, this appears to be a dilemma: either they must accept predestination or God is not omniscient.
Apologists who do not accept predestination may get out of this problem by arguing that God simply knows every possible outcome of every possible decision anyone can make, thus resolving the dilemma.