Authority: Scripture vs. System - N. T. Wright

I have been reading N. T. Wright's book to prepare for my PhD comprehensive exams (which I will take in 2012 or so), and have been really enjoying it. In Chapter 5, The First Sixteen Centuries, Wright looks at how the use and understanding of Scripture changed over the centuries. Starting with allegory, he sees a tension developing: a tension "between authority and interpretation." Here he asks a very good question:

At what point in this process are we forced to conclude that what is really "authoritative" within such an operation is the system of theology or devotion already believed or embraced on other grounds, which is then "discovered" in the text by the interpretative method being used?

found in N. T. Wright, The Last Word (New York: HarperOne, 2005), 66.

So what do you think? How often have I committed that same mistake? How often have you? What can we do, as Wes asked in the comments to another post, do guard ourselves from that mistake?