In the starter dungeon, in the room with the book-cases and the candle-encircled Imp, (here's the "spoiler" if you can call it that) you can fiddle with something (i.e., "click-on") the podium/music-stand/secret-switch to the book-case thing in front of the where the Imp is, and one of the book-cases swing open.
It can be hard to slip through the gap unless you
're a duck, slide and turn a little bit every time you start to "rise up"(climbing mode). This helps to avoid what must be representative of little protrusions and possibly books on Argonian Poetry.
So, if you can't fit through standing, the secret is to crawl. (verified in v2.13)