* (bug 2391) A warning is now shown for invalid ISBN numbers on Special:Booksources.
This could be called from the parser as well, but I'm a little hesitant just yet.
Parser tests currently contain some bogus ISBN numbers which need to be replaced with
legit ones... there's also the possibility that some books really are published with
bad ISBNs... on the other hand, Amazon won't let you search for them. :)
Plus the feedback as you're hitting preview might be nice... but it's also not clear
what's wrong -- that is, that it's an invalid number rather than that you just didn't
do something right. Do we want better feedback?