Sometimes I answer a question like that with a question of my own:

If Almighty God foreknows that something terrible - something He doesn't want - is going to happen, is there any chance that it won't happen?

If anything ever can and does happen that He has not decreed, then He is not Almighty, and not all-knowing. One of the reasons I am so attracted to "Calvinism" is because of its exaltation of God as God!

-Robin