a few weeks ago I got  request for a book that wasn't on my bookshelf (I never had nor owned a copy of that book)

how did that happen?

I've heard of that happening a time or two.  I think the system hiccups from time to time and some folks are caught in the vortex.  I wouldn't worry about it.  If it happens again, you may want to give R&R a heads up so they can look for a problem.

double check the ISBN -- the system can hiccup, but I've never seen it do that. More than likely you posted an ISBN that had a title attached to it that was different than what was on the book (publishers sometimes assign one ISBN to more than one book).

I agree with Teleah its fairly common for two books to have the same ISBNs and if your not careful when posting you can list the wrong book. I once had to, very apologetically, cancel a request because the wrong book was listed and it was on auto request on someones wish list.