Bart Ehrman contends that we cannot trust the Bible because we do not have the original autographs but only copies of copies of copies which are riddled with scribal errors or textual variants. Ehrman (Misquoting Jesus [Kindle] HarperCollins e-books. 2005) discusses how this issue plagued him in his Biblical studies at Moody, Wheaton, and Princeton: 

I kept reverting to my basic question: how does it help us to say that the Bible is the inerrant word of God if in fact, we don’t have the words that God inerrantly inspired, but only the words copied by the scribes— sometimes correctly but sometimes (many times!) incorrectly? What good is it to say that the autographs (i.e., the originals) were inspired? We don’t have the originals! We have only error-ridden copies, and the vast majority of these are centuries removed from the originals and different from them, evidently, in thousands of ways (Kindle location 139).