I have a great deal of experience in this area so, here we go!
- Asking to borrow things without your library card.
- Asking IF there is a fine if you return something late.
Acting surprised when you recieve a fine for returning late.
Being too friggin LOUD!
Ripping pages out of books.
Leaving trash where you please instead of in a trashcan.
Leaving books where you please as opposed to where you got it from.
- Asking "to rent" and not "to borrow" things. This one is so annoying it deserves its own explination. Maybe, this sounds classist, but when you ask "to rent" when you mean "to borrow" you sound like an imbecile. This is only acceptable if english is not your first language, but if it is then may I suggest you BORROW a dictionary before you borrow that other book.
Playing music without headphones (being loud is bad enough, but it take a special kind of asshole to do this).
LEAVING USED CONDOMS IN LIBRARY MATERIALS. YES, THAT REALLY HAPPENED AND THE PERSON JUST SHRUGGED.