I checked my NUS email just now and saw that there was a "Library Circulation Notice" from the NUS Libraries. Scrolling to the bottom, I saw that an RBR book (Reserve Books/Readings - these can be checked out for only 2 hours) was overdue.

Now, since these books are loaned out for only 2 hours, the fine for overdue books is commensurately steep - $1/hr. So when I logged in to my library account, I saw that I had a fine of $25 on my tab.

Currently there is a monitor placed beside the RBR book returning area which will display your name if the book has been detected as returned, and removed from your account. In my case, the problem might have been that, at the early hour I was there, no one else had returned RBR books in the 15 mins or so since I had returned a previous RBR book (since I had returned a book 15 mins before, my name was still displayed on the screen). So maybe the monitor should be tweaked to display the name of the book that has just been returned (it now displays only some cryptic serial number), as well as your name.

I called up the library, and the woman was very nice and helpful about it (they found the book in the end), but she asked me to check my account next time to verify that the book had been returned. Which would make having the monitor there quite redundant.