"Officially, ISBNs should never be reused. However, problems can happen if:
- A publisher improperly reuses an ISBN
- A small or self-publisher mis-registers a book
- An ISBN agency error occurred
- A book was published before 2007 and conversion from ISBN-10 to ISBN-13 created confusion" [Source: ChatGPT]
In 2009, I had plans to use ISBNs to distinguish the books in my personal library. But after scanning some ISBN bar codes with a MacBook app, I discovered some codes were associated with different books (the app also pulled the cover art, so it was easy to spot). Never had the time to find out if the bar code scanning was defective (=did not use the check sum) or these were cases of assignment errors, which "shouldn't happen" but have already happened.
There is a certain type of ignorant developer who reused "unique IDs", I've even seen a database in production use where GUIDs were recycled (no joke).