From a Post on the British Library’s Digital Scholarship Blog:
Ever wondered what that bar code on the back of every book is? It’s an ISBN: an International Standard Book Number. Every modern book published has an ISBN, which uniquely identifies that book, and anyone publishing a book can get an ISBN for it whether an individual or a huge publishing house. It’s a little more complex than that in practice but generally speaking it’s 1 book, 1 ISBN. Right? Right.
While most publishers stick to the rules about never reusing an ISBN, it’s apparently common knowledge in the book trade that ISBNs from old books get reused for newer books, sometimes accidentally (due to a typo), sometimes intentionally (to save money), and that has some tricky consequences.