It's very possible that WOTC had an existing policy to have every 'x' book pulled and checked, and there still to be a problem. I used to work at a couple of printer/binders and I became very familiar with exactly how things are done, and how mistakes are made. A printer will always go over the items that are sent to a customer, in fact if you say "I want to see every 500th book" you will rarely if ever actually see those specific books, the printer may very well check every 500th book, but you usually won't see them, yours get picked from a 'good' pile.
Even when a client sends people down to see the run in progress, things are pre-run before they show up, sometimes if a run was already started and was found to be not good, and the clients are about to arrive, they will 'service' the press claiming a part failure or some such thing, hide all the ones already printed and pretend not to have started at all. Apologizing extensively for the delay, plying danishes and coffee, all the while other people the clients rarely see are running around and fixing the real problem.
If this was just a pallet swap at the binder then it most likely would have been caught as we would be seeing books with the missing section in these doubled up in others. This looks like the printer ran an extra set of these pages by accident and the floorman labeled the pallet wrong, thinking that these were the right ones. I could see this happening during a shift change. OR back at the pre-press area someone makes a duplicate set of plates by accident, and a set gets in the wrong job packet. The guys who actually run the press don't care about what's being printed, they don't look at context, all they really care about is how its being printed. They have a job ticket that says run X amount of these plates, use color X for this plate, and X for this one, etc... If he gets a duplicate packet a day later he won't think twice about it, he may gripe about it, but running a job that looks similar happens, as there may have been a problem during editing/pre-press.
I could go on for ever about what goes on 'behind the scenes' but just take it as a matter of course that as a client you will be lied to every step of the way, and whatever you see isn't what you may think it is.
RX