I am legally entitled to a library card from two different systems (a really great deal for me!). The branches of each that I use most often are located in adjacent cities, and are only a few miles apart.
I got a courtesy email from Library A, that my books were due in three days. But...I returned the books last week. Then I remembered. I had gone to Library B last week, and dumped off ALL my books, including those for Library A.
So I went to Library B in a panic. The nice librarian just chuckled. This happens all the time, he reassured me. People assume that "a library is a library" and return books wherever it's convenient. Especially on Mondays, when Library A was closed--why not just take the books to Library B?
They had a specific procedure for returning books to Library A. Unfortunately, it was cumbersome and took time, so he recommended that I renew my Library A books to avoid fines. I did that, and all was well.