Here is how I describe “proper accident” in my book.
That all men are rational is true in all possible worlds; but that all men have the faculty of laughing is true merely in the actual world, though, indeed, for all men who have ever lived, live now, and will live in the future. The faculty of laughing is, hence, a “proper accident” of a man, situated between essential properties and purely accidental ones like being six feet tall.
My talk of human responsibility should not make one afraid so much that he’ll think God will let him go to hell.
Protection from hell, that is, from infinite suffering for merely potentially infinite humans, is a proper accident of our nature. Hell is possible, just non-actual for all humans at all times.
This is God’s greatest power and highest glory. We depend on Him fully for saving us from errors we absolutely cannot recover from.
At the same time, there is no limit to the suffering we can actually endure until we see the light. Unhappiness can be potentially infinite. Eventually, however, all will come to heaven.