YU's Mendel Gottesman Library's floor layout is notorious. Due to the bizarre architecture, and probably also due to bizarre zoning regulations, the library has a number of "half-floors". For example, there's 2A between floors 2 and 3, and 3A between 3 and 4.
In any case, this is the button array that greets most people in the two standard library elevators (this picture from Elevator No. 2):
However, there is also "Elevator No. 3". This elevator is used by librarians, and requires use of a key to call and ride. With one exception. So as not to potentially cause problems, I won't tell you what the exception is, but feel free to push all the elevator buttons you can find in the library.
I found that I was able to call Elevator No. 3 without a key. When it arrived, I was amazed to see the huge array of floors available to this particular elevator:
Aside from the standard floors, this elevator also included floors GL, GU, 3, 5R and 6! It also had four door open/close buttons. Eager to discover the hidden mysteries of the library I quickly pushed those buttons. But, alas, although I was able to call the elevator, without a key I could not ride it to any of those floors. Then the doors closed and the thing started moving. I knew this meant that a librarian had called the elevator, and knew I might get caught. So I figured that wherever it wound up, I'd just pretend like I belonged there.
When it got to the 6th floor, I said "Hi" to the librarian and started walking. It almost worked. She hesitated, looked at me in a puzzled fashion, and finally asked, "Do you have permission to be here? This floor isn't open to the public." So I played dumb and told her I didn't know that, but before she made me leave could I at least take a picture?
The sixth floor - not so exciting, to be honest. Maybe another time I'll find a way to explore the area more fully.