Quick, what color are Dorothy Gale's shoes in Baum's The Wizard of Oz? Hold that thought, we'll be back to it.
Mark Goodacre has some nice quotes on his 'blog from a 1925 article by E W Lummis, "The Case Against Q." The second quote seems to me to capture the essence of my rejection of Q: Luke knows things that are identifiably Matthean, or, at the very least in this case, don't make sense without knowledge of the Markan narrative--a knowledge Q should lack.
Michael Goulder, while wondering Is Q A Juggernaut points to one such identifiably Matthean tendency in the use of paired animal imagery. One could list more (Goulder, in fact, does so at length; it's the essence of his argument against Q), but doesn't need to: It only takes one, as we shall see in a moment.
Some readers might have known the answer to the question about Dorothy's shoes. Others might have suspected a ruse, so questioned the answer they thought to be correct. Most readers were, of course, unequivocally certain that they knew the answer: Her shoes were red. Most readers are wrong, and it was indeed a ruse. Baum's slippers were silver. The ruby numbers were chosen by MGM to exploit the magic of Technicolor.
The Wizard of Oz has been told and retold time and again, with variations ranging from minor changes, to fundamental alterations of the narrative. Yet of one thing we can be certain: If the slippers are red (or, for that matter, Dorothy auburn-haired), then the author knows of Judy Garland's portrayal of Dorothy. To suggest otherwise seems absurd.
Kloppenborg(pdf) once said of the Farrer-Goulder Hypothesis:
The 2DH is difficult to displace not only because it still serves as an effective compositional hypothesis and because compelling counterevidence has not been produced; but it also allows for a relatively fulsome picture of Jesus and seems to lend support to general theological models concerning the gradual development and articulation of christological, eschatological, and ecclesiological doctrines.
I quote the full passage to avoid robbing it of context, though I must confess that I find the latter portion a baffling requirement. Christology, as we can clearly see from Paul or Hebrews, was more than capable of spiking without a "gradual development." Luke, even if he doesn't know Paul's epistles, clearly knows of Paul and Pauline communities, yet his christology doesn't follow a curve from that. It is presumption, rather than evidence, that demands such a succession.
Which brings us to the obvious analogy (and, like all analogies, it's not perfect, but it is IMO, sufficient), addressing the first half of Kloppenborg's words. That it serves as an "effective compositional hypothesis" or that he finds no "compelling counterevidence," (both points I disagree with, but that's neither here nor there, for the moment) is of little relevance. Luke knows that the slippers were red. The compelling evidence needs to be presented from the other end, else it is fairly demanded that, like the reader here, Luke knows of Judy Garland.