Just to reiterate what Thornir said, that's technically a bindery problem, not a printing problem.
It's not something you would notice on the proofs; and unlikely you'd notice it even on a spot check of the first few books to roll off the bindery.
Definitely the printer's error, they'll be eating the cost to replace those. The sad thing is, there's no tangible way to assess the cost of those errors in terms of lost goodwill.
WOTC will weather the storm; for a small publisher like me, that would have been a public relations DISASTER. Sure, the printer will reprint the books and cover shipping charges, but can you imagine what would happen if 40% of Bad Axe's first hardcover print run had such an error? An error like that could sink a small company, if goodwill with the retailers and customers is broken.
Ugh! My heart goes out to WOTC, and Keith Baker!
Wulf