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).

I've stumbled across 3 or 4 magazines that printed the wrong ISSN in more than one issue. One from the 80s did so in every single issue of it's 20some issue run. It must be true that some books have done so as well, but I don't even check that those are correct.

In my experience this is very very rare. Rare enough that it's practically negligible.