When you say you're one with God, you do the will of God. It kind of removes the question of being fallible. Is God capable of making a mistake?
He must be able to. How did Satan arise? If there was a "war" in heaven, then heaven isn't perfect especially if previously infallible angels on the right-hand of God later became the agents of all that is evil. Either that or St. Peter was on a coffee break and someone else let those guys in the gate originally.