By claiming that they start with Scripture, presuppositionalists make themselves highly susceptible to being unaware of the presuppositions that they bring to Scripture.
Heh. I started down the road to the Catholic Church when I, as a presuppositionalist, began a process of self-examination to try and discern any unbiblical presuppositions I might be unconsciously holding. When I started applying that same procedure to the Reformation (necessarily so, as a theological and intellectual heir of it), the foundations began to crumble: it's no coincidence that humanism (of the Renaissance) and "sola scriptura" both make man the measure of all things (yes, Virginia, that's exactly what "sola scriptura does").
If you are explicitly claiming to start with Scripture, you cannot allow yourself to believe or recognize that you are actually starting with something that you are bringing to Scripture.
Yes, and this makes it oh-so-"fun" to discuss theology with these folks (and I know that I was the exact same way).