Ghost by John Ringo. My wife told the clerk behind the checkout stand that we wanted to return a book. She said "would you mind getting in line?" I said "yes" but my wife said "ok" and did so.
Then, I looked at the new book stands but couldn't find "Ghost" (it was published in October 2005). I looked under Science Fiction but it wasn't there. Their "book specialist" looked it up and it turned out that "Ghost" was under "novels" and not under "Science Fiction". We both looked and I found it; the book was misshelved. This didn't take long and I was able to take the book to the check out stand just as the clerk got to my wife. We did the swap and ended up getting $1.30 back...
It turned out that a couple of clerks had gone to lunch and left this one young woman alone at the cash registers when "a thundering herd" of customers had come in. (She didn't say this, this is something we used to say when I worked at the college library.)