One mistake is to think that the doctrine of Original Sin is empirically verifiable. I have seen this thought attributed to Reinhold Niebuhr. (If someone can supply a reference for me with exact bibliographical data, I would be much obliged.) I could easily be mistaken, but I believe I have encountered the thought in Kierkegaard as well. (Anyone have a reference?) G. K. Chesterton says essentially the same thing. See my post, Is Sin a Fact? A Passage from Chesterton Examined. Chesterton thinks that sin, and indeed original sin, is a plain fact for all to see. That is simply not the case as I argue.
The opposite mistake is to think that Original Sin is obviously false and empirically refutable by evolutionary biology. Thus: no Fall because no original biologically human parents. As if the doctrine of the Fall 'stands or falls' with the truth of a passage in Genesis literally interpreted. I lately explained why I think that is a mistake, and indeed a rather stupid one, though my explanation left something to be desired. (I am working on a longer post on the Fall as we speak.)
So on the one hand we have those who maintain that the doctrine of Original Sin is true as a matter of empirical fact, and on the other we have those who maintain that it is false as a matter of empirical fact. On both sides we find very intelligent people. I take this disagreement as further evidence that we are indeed fallen beings, 'noetically wretched,' to coin a phrase, beings whose reason is so infirm and befouled that we can even argue about such a thing. And of course my own view, according to which OS is neither empirically true nor empirically false, is just another voice added to the cacophony of conflicting voices, though, as it seems to me, it has more merit than the other two.
So we are in deep caca, intellectually, morally, and in every which way -- which is why I believe in 'something like' Original Sin. Our condition is a fallen one, and indeed one that is (i) universal in that it applies to everyone, and (ii) unameliorable by anything we can do, individually or collectively. You say I need to justify these bold claims? I agree! But it's Saturday night, the sun is setting, and it's time to close up shop for the day. So, invoking the blogospheric privilege deriving from the truth that brevity is the soul of blog, I simply punch the clock.