Consider this dialogue about the modal ontological argument.
T: Surely, you think it's possible that there is a God, surely. I mean, maybe he exists, doncha think. I mean, you aren't one of those dogmatic atheists, are you.
A: Yes, of course, I admit that it's possible that God exists.
T: Gotcha! Gotcha! Gotcha! Since the existence of God is either necessarily true or necessarily false, the S5 axiom says that if a necessary truth claim is possibly true, it must be necessarily true. Since you have admitted that God possibly exists, you must therefore conclude that God necessarily exists!!!
A: Uh, could we maybe restrict the accessibility relation or something?
Bill Vallicella is dealing with an atheist argument on his site with much the same problems.