[rmunn]

I've been cataloguing my books using the ISBN to look them up, and I think I ran into that situation a few times, maybe about 0.2% of all the books I catalogued. (That is, the ISBN search on openlibrary.org returned multiple clearly-different books for the ISBN I searched for). I didn't pay much attention to it so I can't tell you which ISBNs were duplicates, but I've definitely seen it happen.

But there is at least one case where it was on purpose. There's a set of reading primers from the UK called the Biff, Chip and Kipper books. We acquired a whole set of them at a garage sale, and when I went to enter them into my catalogue, I discovered that the publisher had assigned just one ISBN to the whole series. Which quite annoyed me when I discovered it. (I ended up just not cataloguing those books, because I didn't want to type the titles, author, copyright date, etc. in by hand for 50+ tiny books).